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KVM: MMU: fast page fault

PATCH In review Xiao Guangrong 5 participants 21 messages Jun 20, 2012 – Jul 11, 2012

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Xiao Guangrong

author
#0 lore raw

v7 cover KVM: MMU: fast page fault

Changlog:
- always atomicly update the spte if it can be updated out of mmu-lock
- rename spte_can_be_writable() to spte_is_locklessly_modifiable()
- cleanup and comment spte_write_protect()

Performance result:
(The benchmark can be found at: http://www.spinics.net/lists/kvm/msg73011.html)

before after
Run 10 times, Avg time: 538233957 ns. 249809853 ns. +53.6%

author
#1 lore raw

PATCH v7 1/10 KVM: MMU: return bool in __rmap_write_protect

The reture value of __rmap_write_protect is either 1 or 0, use true/false instead of these

Signed-off-by Xiao Guangrong <[email protected]>
---
 arch/x86/kvm/mmu.c |   13 +++++++------
 1 files changed, 7 insertions(+), 6 deletions(-)
arch/x86/kvm/mmu.c +7 -6
index 24dd43d..fad5c91 100644
@@ -1051,11 +1051,12 @@ static void drop_spte(struct kvm *kvm, u64 *sptep)
 		rmap_remove(kvm, sptep);
 }
 
-static int __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level)
+static bool
+__rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level)
 {
 	u64 *sptep;
 	struct rmap_iterator iter;
-	int write_protected = 0;
+	bool write_protected = false;
 
 	for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
 		BUG_ON(!(*sptep & PT_PRESENT_MASK));

@@ -1076,7 +1077,7 @@ static int __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level
 			sptep = rmap_get_first(*rmapp, &iter);
 		}
 
-		write_protected = 1;
+		write_protected = true;
 	}
 
 	return write_protected;

@@ -1107,12 +1108,12 @@ void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
 	}
 }
 
-static int rmap_write_protect(struct kvm *kvm, u64 gfn)
+static bool rmap_write_protect(struct kvm *kvm, u64 gfn)
 {
 	struct kvm_memory_slot *slot;
 	unsigned long *rmapp;
 	int i;
-	int write_protected = 0;
+	bool write_protected = false;
 
 	slot = gfn_to_memslot(kvm, gfn);
 

@@ -1703,7 +1704,7 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
 
 	kvm_mmu_pages_init(parent, &parents, &pages);
 	while (mmu_unsync_walk(parent, &pages)) {
-		int protected = 0;
+		bool protected = false;
 
 		for_each_sp(pages, sp, parents, i)
 			protected |= rmap_write_protect(vcpu->kvm, sp->gfn);
Signature · 1 line
1.7.7.6
author
#2 lore raw

PATCH v7 2/10 KVM: MMU: abstract spte write-protect

In reply to Xiao Guangrong

Introduce a common function to abstract spte write-protect to cleanup the code

Signed-off-by Xiao Guangrong <[email protected]>
---
 arch/x86/kvm/mmu.c |   58 +++++++++++++++++++++++++++------------------------
 1 files changed, 31 insertions(+), 27 deletions(-)
arch/x86/kvm/mmu.c +31 -27
index fad5c91..334dc54 100644
@@ -1051,36 +1051,48 @@ static void drop_spte(struct kvm *kvm, u64 *sptep)
 		rmap_remove(kvm, sptep);
 }
 
+/* Return true if the spte is dropped. */
+static bool spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush)
+{
+	u64 spte = *sptep;
+
+	if (!is_writable_pte(spte))
+		return false;
+
+	rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);
+
+	*flush |= true;
+	if (is_large_pte(spte)) {
+		WARN_ON(page_header(__pa(sptep))->role.level ==
+		       PT_PAGE_TABLE_LEVEL);
+		drop_spte(kvm, sptep);
+		--kvm->stat.lpages;
+		return true;
+	}
+
+	spte = spte & ~PT_WRITABLE_MASK;
+	mmu_spte_update(sptep, spte);
+	return false;
+}
+
 static bool
 __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level)
 {
 	u64 *sptep;
 	struct rmap_iterator iter;
-	bool write_protected = false;
+	bool flush = false;
 
 	for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
 		BUG_ON(!(*sptep & PT_PRESENT_MASK));
-		rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);
-
-		if (!is_writable_pte(*sptep)) {
-			sptep = rmap_get_next(&iter);
-			continue;
-		}
-
-		if (level == PT_PAGE_TABLE_LEVEL) {
-			mmu_spte_update(sptep, *sptep & ~PT_WRITABLE_MASK);
-			sptep = rmap_get_next(&iter);
-		} else {
-			BUG_ON(!is_large_pte(*sptep));
-			drop_spte(kvm, sptep);
-			--kvm->stat.lpages;
+		if (spte_write_protect(kvm, sptep, &flush)) {
 			sptep = rmap_get_first(*rmapp, &iter);
+			continue;
 		}
 
-		write_protected = true;
+		sptep = rmap_get_next(&iter);
 	}
 
-	return write_protected;
+	return flush;
 }
 
 /**

@@ -3888,6 +3900,7 @@ int kvm_mmu_setup(struct kvm_vcpu *vcpu)
 void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
 {
 	struct kvm_mmu_page *sp;
+	bool flush = false;
 
 	list_for_each_entry(sp, &kvm->arch.active_mmu_pages, link) {
 		int i;

@@ -3902,16 +3915,7 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
 			      !is_last_spte(pt[i], sp->role.level))
 				continue;
 
-			if (is_large_pte(pt[i])) {
-				drop_spte(kvm, &pt[i]);
-				--kvm->stat.lpages;
-				continue;
-			}
-
-			/* avoid RMW */
-			if (is_writable_pte(pt[i]))
-				mmu_spte_update(&pt[i],
-						pt[i] & ~PT_WRITABLE_MASK);
+			spte_write_protect(kvm, &pt[i], &flush);
 		}
 	}
 	kvm_flush_remote_tlbs(kvm);
Signature · 1 line
1.7.7.6
#3 lore raw

Re: [PATCH v7 02/10] KVM: MMU: abstract spte write-protect

Quoted 4 lines · On Wed, 20 Jun 2012 15:57:15 +0800 Xiao Guangrong…

Introduce a common function to abstract spte write-protect to cleanup the code

Signed-off-by Xiao Guangrong <[email protected]>

...

Quoted 9 lines
arch/x86/kvm/mmu.c +9 -0
+/* Return true if the spte is dropped. */
+static bool spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush)
+{
+	u64 spte = *sptep;
+
+	if (!is_writable_pte(spte))
+		return false;
+
+	rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);

...

Quoted 5 lines
arch/x86/kvm/mmu.c +0 -1
@@ -3902,16 +3915,7 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
 			      !is_last_spte(pt[i], sp->role.level))
 				continue;
 
-			if (is_large_pte(pt[i])) {

arch/x86/kvm/mmu.c void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot) · line 3918

Quoted 10 lines
arch/x86/kvm/mmu.c +1 -9
-				drop_spte(kvm, &pt[i]);
-				--kvm->stat.lpages;
-				continue;
-			}
-
-			/* avoid RMW */
-			if (is_writable_pte(pt[i]))
-				mmu_spte_update(&pt[i],
-						pt[i] & ~PT_WRITABLE_MASK);
+			spte_write_protect(kvm, &pt[i], &flush);

Adding rmap_printk() here seems wrong.

If you think it is not a problem, please explain why you think so in the changelog.

Thanks,
Takuya

author
#4 lore raw

Re: [PATCH v7 02/10] KVM: MMU: abstract spte write-protect

Quoted 26 lines · Takuya Yoshikawa
Quoted 4 lines · On Wed, 20 Jun 2012 15:57:15 +0800 Xiao Guangrong…

Introduce a common function to abstract spte write-protect to cleanup the code

Signed-off-by Xiao Guangrong <[email protected]>

...

Quoted 9 lines
arch/x86/kvm/mmu.c +9 -0
+/* Return true if the spte is dropped. */
+static bool spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush)
+{
+	u64 spte = *sptep;
+
+	if (!is_writable_pte(spte))
+		return false;
+
+	rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);

...

Quoted 4 lines
arch/x86/kvm/mmu.c +0 -0
@@ -3902,16 +3915,7 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
 			      !is_last_spte(pt[i], sp->role.level))
 				continue;
Quoted 14 lines

arch/x86/kvm/mmu.c void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot) · line 3918

Quoted 11 lines
arch/x86/kvm/mmu.c +1 -10
-			if (is_large_pte(pt[i])) {
-				drop_spte(kvm, &pt[i]);
-				--kvm->stat.lpages;
-				continue;
-			}
-
-			/* avoid RMW */
-			if (is_writable_pte(pt[i]))
-				mmu_spte_update(&pt[i],
-						pt[i] & ~PT_WRITABLE_MASK);
+			spte_write_protect(kvm, &pt[i], &flush);

Adding rmap_printk() here seems wrong.

Strange! Why do you think it is wrong? It is just debug code.

Quoted 2 lines

If you think it is not a problem, please explain why you think so in the changelog.

It is a from the first place and it is used to debug and not compiled at all.

#5 lore raw

Re: [PATCH v7 02/10] KVM: MMU: abstract spte write-protect

Quoted 1 line · On Wed, 20 Jun 2012 17:11:06 +0800 Xiao Guangrong…

Strange! Why do you think it is wrong? It is just debug code.

kvm_mmu_slot_remove_write_access() does not use rmap but the debug code says:

	rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);
Quoted 5 lines
Quoted 2 lines

If you think it is not a problem, please explain why you think so in the changelog.

It is a from the first place and it is used to debug and not compiled at all.

It was not in kvm_mmu_slot_remove_write_access() before, no?

This patch says that the write protection code becomes commonly usable function, but it still has rmap_write_protect specific debug code in it; using it in kvm_mmu_slot_remove_write_access(), which is not at all related to rmap_write_protect, is strange.

As you say, this is just debug code and does not have any practical problem. But randomly putting debug code is not a good thing.

Thanks,
Takuya

author
#6 lore raw

Re: [PATCH v7 02/10] KVM: MMU: abstract spte write-protect

Quoted 7 lines · Takuya Yoshikawa
Quoted 1 line · On Wed, 20 Jun 2012 17:11:06 +0800 Xiao Guangrong…

Strange! Why do you think it is wrong? It is just debug code.

kvm_mmu_slot_remove_write_access() does not use rmap but the debug code says:

	rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);

It is not a problem since all sptes which are pointing to gfn is existed in rmap.

Quoted 9 lines
Quoted 6 lines
Quoted 3 lines

If you think it is not a problem, please explain why you think so in the changelog.

It is a from the first place and it is used to debug and not compiled at all.

It was not in kvm_mmu_slot_remove_write_access() before, no?

Quoted 8 lines

This patch says that the write protection code becomes commonly usable function, but it still has rmap_write_protect specific debug code in it; using it in kvm_mmu_slot_remove_write_access(), which is not at all related to rmap_write_protect, is strange.

As you say, this is just debug code and does not have any practical problem. But randomly putting debug code is not a good thing.

Again, "rmap" does not break the logic, the spte we handle in this function must be in rmap.

#7 lore raw

Re: [PATCH v7 02/10] KVM: MMU: abstract spte write-protect

Quoted 2 lines · On Wed, 20 Jun 2012 21:21:07 +0800 Xiao Guangrong…

Again, "rmap" does not break the logic, the spte we handle in this function must be in rmap.

I'm not saying whether this breaks some logic or not.

rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep); helps developers by saying "rmap_write_protect() protected ...."

For another function, the message should be "another_func_name: spte ..." But OK, I won't argue about this anymore.

We can change the debug message later if needed.

Thanks,
Takuya

author
#8 lore raw

Re: [PATCH v7 02/10] KVM: MMU: abstract spte write-protect

Quoted 1 line · Takuya Yoshikawa

We can change the debug message later if needed.

Actually, i am going to use tracepoint instead of
these debug code.

#9 lore raw

Re: [PATCH v7 02/10] KVM: MMU: abstract spte write-protect

Quoted 4 lines · On Thu, 21 Jun 2012 09:48:05 +0800 Xiao Guangrong…
Quoted 1 line

We can change the debug message later if needed.

Actually, i am going to use tracepoint instead of
these debug code.

That's very nice!

Then, please change the trace log to correspond to the new spte_write_protect name at that time.

Thank you, Xiao.

Takuya

#10 lore raw

Re: [PATCH v7 02/10] KVM: MMU: abstract spte write-protect

In reply to Xiao Guangrong
Quoted 8 lines · Xiao Guangrong
Quoted 1 line · Takuya Yoshikawa

We can change the debug message later if needed.

Actually, i am going to use tracepoint instead of
these debug code.

Yes, these should be in the kvmmmu namespace.

Signature · 1 line
error compiling committee.c: too many arguments to function
author
#11 lore raw

PATCH v7 3/10 KVM: MMU: cleanup spte_write_protect

In reply to Xiao Guangrong

Use __drop_large_spte to cleanup this function and comment spte_write_protect

Signed-off-by Xiao Guangrong <[email protected]>
---
 arch/x86/kvm/mmu.c |   45 +++++++++++++++++++++++++++++----------------
 1 files changed, 29 insertions(+), 16 deletions(-)
arch/x86/kvm/mmu.c +29 -16
index 334dc54..a0ea746 100644
@@ -1051,7 +1051,33 @@ static void drop_spte(struct kvm *kvm, u64 *sptep)
 		rmap_remove(kvm, sptep);
 }
 
-/* Return true if the spte is dropped. */
+
+static bool __drop_large_spte(struct kvm *kvm, u64 *sptep)
+{
+	if (is_large_pte(*sptep)) {
+		WARN_ON(page_header(__pa(sptep))->role.level ==
+			PT_PAGE_TABLE_LEVEL);
+		drop_spte(kvm, sptep);
+		--kvm->stat.lpages;
+		return true;
+	}
+
+	return false;
+}
+
+static void drop_large_spte(struct kvm_vcpu *vcpu, u64 *sptep)
+{
+	if (__drop_large_spte(vcpu->kvm, sptep))
+		kvm_flush_remote_tlbs(vcpu->kvm);
+}
+
+/*
+ * Write-protect on the specified @sptep due to dirty page logging or
+ * protecting shadow page table. @flush indicates whether tlb need be
+ * flushed.
+ *
+ * Return true if the spte is dropped.
+ */
 static bool spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush)
 {
 	u64 spte = *sptep;

@@ -1062,13 +1088,9 @@ static bool spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush)
 	rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);
 
 	*flush |= true;
-	if (is_large_pte(spte)) {
-		WARN_ON(page_header(__pa(sptep))->role.level ==
-		       PT_PAGE_TABLE_LEVEL);
-		drop_spte(kvm, sptep);
-		--kvm->stat.lpages;
+
+	if (__drop_large_spte(kvm, sptep))
 		return true;
-	}
 
 	spte = spte & ~PT_WRITABLE_MASK;
 	mmu_spte_update(sptep, spte);

@@ -1881,15 +1903,6 @@ static void link_shadow_page(u64 *sptep, struct kvm_mmu_page *sp)
 	mmu_spte_set(sptep, spte);
 }
 
-static void drop_large_spte(struct kvm_vcpu *vcpu, u64 *sptep)
-{
-	if (is_large_pte(*sptep)) {
-		drop_spte(vcpu->kvm, sptep);
-		--vcpu->kvm->stat.lpages;
-		kvm_flush_remote_tlbs(vcpu->kvm);
-	}
-}
-
 static void validate_direct_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 				   unsigned direct_access)
 {
Signature · 1 line
1.7.7.6
author
#12 lore raw

PATCH v7 4/10 KVM: VMX: export PFEC.P bit on ept

In reply to Xiao Guangrong

Export the present bit of page fault error code, the later patch will use it

Signed-off-by Xiao Guangrong <[email protected]>
---
 arch/x86/kvm/vmx.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)
arch/x86/kvm/vmx.c +8 -1
index eeeb4a2..a44c649 100644
@@ -4829,6 +4829,7 @@ static int handle_ept_violation(struct kvm_vcpu *vcpu)
 {
 	unsigned long exit_qualification;
 	gpa_t gpa;
+	u32 error_code;
 	int gla_validity;
 
 	exit_qualification = vmcs_readl(EXIT_QUALIFICATION);

@@ -4853,7 +4854,13 @@ static int handle_ept_violation(struct kvm_vcpu *vcpu)
 
 	gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS);
 	trace_kvm_page_fault(gpa, exit_qualification);
-	return kvm_mmu_page_fault(vcpu, gpa, exit_qualification & 0x3, NULL, 0);
+
+	/* It is a write fault? */
+	error_code = exit_qualification & (1U << 1);
+	/* ept page table is present? */
+	error_code |= (exit_qualification >> 3) & 0x1;
+
+	return kvm_mmu_page_fault(vcpu, gpa, error_code, NULL, 0);
 }
 
 static u64 ept_rsvd_mask(u64 spte, int level)
Signature · 1 line
1.7.7.6
author
#13 lore raw

PATCH v7 5/10 KVM: MMU: fold tlb flush judgement into mmu_spte_update

In reply to Xiao Guangrong

mmu_spte_update() is the common function, we can easily audit the path

Signed-off-by Xiao Guangrong <[email protected]>
---
 arch/x86/kvm/mmu.c |   33 ++++++++++++++++++++-------------
 1 files changed, 20 insertions(+), 13 deletions(-)
arch/x86/kvm/mmu.c +20 -13
index a0ea746..6ebbb7a 100644
@@ -478,15 +478,24 @@ static void mmu_spte_set(u64 *sptep, u64 new_spte)
 
 /* Rules for using mmu_spte_update:
  * Update the state bits, it means the mapped pfn is not changged.
+ *
+ * Whenever we overwrite a writable spte with a read-only one we
+ * should flush remote TLBs. Otherwise rmap_write_protect
+ * will find a read-only spte, even though the writable spte
+ * might be cached on a CPU's TLB, the return value indicates this
+ * case.
  */
-static void mmu_spte_update(u64 *sptep, u64 new_spte)
+static bool mmu_spte_update(u64 *sptep, u64 new_spte)
 {
 	u64 mask, old_spte = *sptep;
+	bool ret = false;
 
 	WARN_ON(!is_rmap_spte(new_spte));
 
-	if (!is_shadow_present_pte(old_spte))
-		return mmu_spte_set(sptep, new_spte);
+	if (!is_shadow_present_pte(old_spte)) {
+		mmu_spte_set(sptep, new_spte);
+		return ret;
+	}
 
 	new_spte |= old_spte & shadow_dirty_mask;
 

@@ -499,13 +508,18 @@ static void mmu_spte_update(u64 *sptep, u64 new_spte)
 	else
 		old_spte = __update_clear_spte_slow(sptep, new_spte);
 
+	if (is_writable_pte(old_spte) && !is_writable_pte(new_spte))
+		ret = true;
+
 	if (!shadow_accessed_mask)
-		return;
+		return ret;
 
 	if (spte_is_bit_cleared(old_spte, new_spte, shadow_accessed_mask))
 		kvm_set_pfn_accessed(spte_to_pfn(old_spte));
 	if (spte_is_bit_cleared(old_spte, new_spte, shadow_dirty_mask))
 		kvm_set_pfn_dirty(spte_to_pfn(old_spte));
+
+	return ret;
 }
 
 /*

@@ -2271,7 +2285,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 		    gfn_t gfn, pfn_t pfn, bool speculative,
 		    bool can_unsync, bool host_writable)
 {
-	u64 spte, entry = *sptep;
+	u64 spte;
 	int ret = 0;
 
 	if (set_mmio_spte(sptep, gfn, pfn, pte_access))

@@ -2349,14 +2363,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 		mark_page_dirty(vcpu->kvm, gfn);
 
 set_pte:
-	mmu_spte_update(sptep, spte);
-	/*
-	 * If we overwrite a writable spte with a read-only one we
-	 * should flush remote TLBs. Otherwise rmap_write_protect
-	 * will find a read-only spte, even though the writable spte
-	 * might be cached on a CPU's TLB.
-	 */
-	if (is_writable_pte(entry) && !is_writable_pte(*sptep))
+	if (mmu_spte_update(sptep, spte))
 		kvm_flush_remote_tlbs(vcpu->kvm);
 done:
 	return ret;
Signature · 1 line
1.7.7.6
author
#14 lore raw

PATCH v7 6/10 KVM: MMU: introduce SPTE_MMU_WRITEABLE bit

In reply to Xiao Guangrong

This bit indicates whether the spte can be writable on MMU, that means the corresponding gpte is writable and the corresponding gfn is not protected by shadow page protection

In the later path, SPTE_MMU_WRITEABLE will indicates whether the spte can be locklessly updated

Signed-off-by Xiao Guangrong <[email protected]>
---
 arch/x86/kvm/mmu.c |   57 ++++++++++++++++++++++++++++++++++-----------------
 1 files changed, 38 insertions(+), 19 deletions(-)
arch/x86/kvm/mmu.c +38 -19
index 6ebbb7a..c3ca6e8 100644
@@ -145,7 +145,8 @@ module_param(dbg, bool, 0644);
 #define CREATE_TRACE_POINTS
 #include "mmutrace.h"
 
-#define SPTE_HOST_WRITEABLE (1ULL << PT_FIRST_AVAIL_BITS_SHIFT)
+#define SPTE_HOST_WRITEABLE	(1ULL << PT_FIRST_AVAIL_BITS_SHIFT)
+#define SPTE_MMU_WRITEABLE	(1ULL << (PT_FIRST_AVAIL_BITS_SHIFT + 1))
 
 #define SHADOW_PT_INDEX(addr, level) PT64_INDEX(addr, level)
 

@@ -1085,34 +1086,51 @@ static void drop_large_spte(struct kvm_vcpu *vcpu, u64 *sptep)
 		kvm_flush_remote_tlbs(vcpu->kvm);
 }
 
+static bool spte_is_locklessly_modifiable(u64 spte)
+{
+	return !(~spte & (SPTE_HOST_WRITEABLE | SPTE_MMU_WRITEABLE));
+}
+
 /*
- * Write-protect on the specified @sptep due to dirty page logging or
- * protecting shadow page table. @flush indicates whether tlb need be
- * flushed.
+ * Write-protect on the specified @sptep, @pt_protect indicates whether
+ * spte writ-protection is caused by protecting shadow page table.
+ * @flush indicates whether tlb need be flushed.
+ *
+ * Note: write protection is difference between drity logging and spte
+ * protection:
+ * - for dirty logging, the spte can be set to writable at anytime if
+ *   its dirty bitmap is properly set.
+ * - for spte protection, the spte can be writable only after unsync-ing
+ *   shadow page.
  *
  * Return true if the spte is dropped.
  */
-static bool spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush)
+static bool
+spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush, bool pt_protect)
 {
 	u64 spte = *sptep;
 
-	if (!is_writable_pte(spte))
+	if (!is_writable_pte(spte) &&
+	      !(pt_protect && spte_is_locklessly_modifiable(spte)))
 		return false;
 
 	rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);
 
-	*flush |= true;
-
-	if (__drop_large_spte(kvm, sptep))
+	if (__drop_large_spte(kvm, sptep)) {
+		*flush |= true;
 		return true;
+	}
 
+	if (pt_protect)
+		spte &= ~SPTE_MMU_WRITEABLE;
 	spte = spte & ~PT_WRITABLE_MASK;
-	mmu_spte_update(sptep, spte);
+
+	*flush |= mmu_spte_update(sptep, spte);
 	return false;
 }
 
-static bool
-__rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level)
+static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,
+				 int level, bool pt_protect)
 {
 	u64 *sptep;
 	struct rmap_iterator iter;

@@ -1120,7 +1138,7 @@ __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level)
 
 	for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
 		BUG_ON(!(*sptep & PT_PRESENT_MASK));
-		if (spte_write_protect(kvm, sptep, &flush)) {
+		if (spte_write_protect(kvm, sptep, &flush, pt_protect)) {
 			sptep = rmap_get_first(*rmapp, &iter);
 			continue;
 		}

@@ -1149,7 +1167,7 @@ void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
 
 	while (mask) {
 		rmapp = &slot->rmap[gfn_offset + __ffs(mask)];
-		__rmap_write_protect(kvm, rmapp, PT_PAGE_TABLE_LEVEL);
+		__rmap_write_protect(kvm, rmapp, PT_PAGE_TABLE_LEVEL, false);
 
 		/* clear the first set bit */
 		mask &= mask - 1;

@@ -1168,7 +1186,7 @@ static bool rmap_write_protect(struct kvm *kvm, u64 gfn)
 	for (i = PT_PAGE_TABLE_LEVEL;
 	     i < PT_PAGE_TABLE_LEVEL + KVM_NR_PAGE_SIZES; ++i) {
 		rmapp = __gfn_to_rmap(gfn, i, slot);
-		write_protected |= __rmap_write_protect(kvm, rmapp, i);
+		write_protected |= __rmap_write_protect(kvm, rmapp, i, true);
 	}
 
 	return write_protected;

@@ -2299,8 +2317,10 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 		spte |= shadow_x_mask;
 	else
 		spte |= shadow_nx_mask;
+
 	if (pte_access & ACC_USER_MASK)
 		spte |= shadow_user_mask;
+
 	if (level > PT_PAGE_TABLE_LEVEL)
 		spte |= PT_PAGE_SIZE_MASK;
 	if (tdp_enabled)

@@ -2325,7 +2345,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 			goto done;
 		}
 
-		spte |= PT_WRITABLE_MASK;
+		spte |= PT_WRITABLE_MASK | SPTE_MMU_WRITEABLE;
 
 		if (!vcpu->arch.mmu.direct_map
 		    && !(pte_access & ACC_WRITE_MASK)) {

@@ -2354,8 +2374,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 				 __func__, gfn);
 			ret = 1;
 			pte_access &= ~ACC_WRITE_MASK;
-			if (is_writable_pte(spte))
-				spte &= ~PT_WRITABLE_MASK;
+			spte &= ~(PT_WRITABLE_MASK | SPTE_MMU_WRITEABLE);
 		}
 	}
 

@@ -3935,7 +3954,7 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
 			      !is_last_spte(pt[i], sp->role.level))
 				continue;
 
-			spte_write_protect(kvm, &pt[i], &flush);
+			spte_write_protect(kvm, &pt[i], &flush, false);
 		}
 	}
 	kvm_flush_remote_tlbs(kvm);
Signature · 1 line
1.7.7.6
author
#15 lore raw

PATCH v7 7/10 KVM: MMU: fast path of handling guest page fault

In reply to Xiao Guangrong

If the the present bit of page fault error code is set, it indicates the shadow page is populated on all levels, it means what we do is only modify the access bit which can be done out of mmu-lock

Currently, in order to simplify the code, we only fix the page fault caused by write-protect on the fast path

Signed-off-by Xiao Guangrong <[email protected]>
---
 arch/x86/kvm/mmu.c |  144 +++++++++++++++++++++++++++++++++++++++++++++------
 1 files changed, 127 insertions(+), 17 deletions(-)
arch/x86/kvm/mmu.c +127 -17
index c3ca6e8..37c0230 100644
@@ -445,8 +445,22 @@ static bool __check_direct_spte_mmio_pf(u64 spte)
 }
 #endif
 
+static bool spte_is_locklessly_modifiable(u64 spte)
+{
+	return !(~spte & (SPTE_HOST_WRITEABLE | SPTE_MMU_WRITEABLE));
+}
+
 static bool spte_has_volatile_bits(u64 spte)
 {
+	/*
+	 * Always atomicly update spte if it can be updated
+	 * out of mmu-lock, it can ensure dirty bit is not lost,
+	 * also, it can help us to get a stable is_writable_pte()
+	 * to ensure tlb flush is not missed.
+	 */
+	if (spte_is_locklessly_modifiable(spte))
+		return true;
+
 	if (!shadow_accessed_mask)
 		return false;
 

@@ -488,7 +502,7 @@ static void mmu_spte_set(u64 *sptep, u64 new_spte)
  */
 static bool mmu_spte_update(u64 *sptep, u64 new_spte)
 {
-	u64 mask, old_spte = *sptep;
+	u64 old_spte = *sptep;
 	bool ret = false;
 
 	WARN_ON(!is_rmap_spte(new_spte));

@@ -498,17 +512,16 @@ static bool mmu_spte_update(u64 *sptep, u64 new_spte)
 		return ret;
 	}
 
-	new_spte |= old_spte & shadow_dirty_mask;
-
-	mask = shadow_accessed_mask;
-	if (is_writable_pte(old_spte))
-		mask |= shadow_dirty_mask;
-
-	if (!spte_has_volatile_bits(old_spte) || (new_spte & mask) == mask)
+	if (!spte_has_volatile_bits(old_spte))
 		__update_clear_spte_fast(sptep, new_spte);
 	else
 		old_spte = __update_clear_spte_slow(sptep, new_spte);
 
+	/*
+	 * For the spte updated out of mmu-lock is safe, since
+	 * we always atomicly update it, see the comments in
+	 * spte_has_volatile_bits().
+	 */
 	if (is_writable_pte(old_spte) && !is_writable_pte(new_spte))
 		ret = true;
 

@@ -1086,11 +1099,6 @@ static void drop_large_spte(struct kvm_vcpu *vcpu, u64 *sptep)
 		kvm_flush_remote_tlbs(vcpu->kvm);
 }
 
-static bool spte_is_locklessly_modifiable(u64 spte)
-{
-	return !(~spte & (SPTE_HOST_WRITEABLE | SPTE_MMU_WRITEABLE));
-}
-
 /*
  * Write-protect on the specified @sptep, @pt_protect indicates whether
  * spte writ-protection is caused by protecting shadow page table.

@@ -2679,18 +2687,114 @@ exit:
 	return ret;
 }
 
+static bool page_fault_can_be_fast(struct kvm_vcpu *vcpu, u32 error_code)
+{
+	/*
+	 * #PF can be fast only if the shadow page table is present and it
+	 * is caused by write-protect, that means we just need change the
+	 * W bit of the spte which can be done out of mmu-lock.
+	 */
+	if (!(error_code & PFERR_PRESENT_MASK) ||
+	      !(error_code & PFERR_WRITE_MASK))
+		return false;
+
+	return true;
+}
+
+static bool
+fast_pf_fix_direct_spte(struct kvm_vcpu *vcpu, u64 *sptep, u64 spte)
+{
+	struct kvm_mmu_page *sp = page_header(__pa(sptep));
+	gfn_t gfn;
+
+	WARN_ON(!sp->role.direct);
+
+	/*
+	 * The gfn of direct spte is stable since it is calculated
+	 * by sp->gfn.
+	 */
+	gfn = kvm_mmu_page_get_gfn(sp, sptep - sp->spt);
+
+	if (cmpxchg64(sptep, spte, spte | PT_WRITABLE_MASK) == spte)
+		mark_page_dirty(vcpu->kvm, gfn);
+
+	return true;
+}
+
+/*
+ * Return value:
+ * - true: let the vcpu to access on the same address again.
+ * - false: let the real page fault path to fix it.
+ */
+static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level,
+			    u32 error_code)
+{
+	struct kvm_shadow_walk_iterator iterator;
+	bool ret = false;
+	u64 spte = 0ull;
+
+	if (!page_fault_can_be_fast(vcpu, error_code))
+		return false;
+
+	walk_shadow_page_lockless_begin(vcpu);
+	for_each_shadow_entry_lockless(vcpu, gva, iterator, spte)
+		if (!is_shadow_present_pte(spte) || iterator.level < level)
+			break;
+
+	/*
+	 * If the mapping has been changed, let the vcpu fault on the
+	 * same address again.
+	 */
+	if (!is_rmap_spte(spte)) {
+		ret = true;
+		goto exit;
+	}
+
+	if (!is_last_spte(spte, level))
+		goto exit;
+
+	/*
+	 * Check if it is a spurious fault caused by TLB lazily flushed.
+	 *
+	 * Need not check the access of upper level table entries since
+	 * they are always ACC_ALL.
+	 */
+	 if (is_writable_pte(spte)) {
+		ret = true;
+		goto exit;
+	}
+
+	/*
+	 * Currently, to simplify the code, only the spte write-protected
+	 * by dirty-log can be fast fixed.
+	 */
+	if (!spte_is_locklessly_modifiable(spte))
+		goto exit;
+
+	/*
+	 * Currently, fast page fault only works for direct mapping since
+	 * the gfn is not stable for indirect shadow page.
+	 * See Documentation/virtual/kvm/locking.txt to get more detail.
+	 */
+	ret = fast_pf_fix_direct_spte(vcpu, iterator.sptep, spte);
+exit:
+	walk_shadow_page_lockless_end(vcpu);
+
+	return ret;
+}
+
 static bool try_async_pf(struct kvm_vcpu *vcpu, bool prefault, gfn_t gfn,
 			 gva_t gva, pfn_t *pfn, bool write, bool *writable);
 
-static int nonpaging_map(struct kvm_vcpu *vcpu, gva_t v, int write, gfn_t gfn,
-			 bool prefault)
+static int nonpaging_map(struct kvm_vcpu *vcpu, gva_t v, u32 error_code,
+			 gfn_t gfn, bool prefault)
 {
 	int r;
 	int level;
 	int force_pt_level;
 	pfn_t pfn;
 	unsigned long mmu_seq;
-	bool map_writable;
+	bool map_writable, write = error_code & PFERR_WRITE_MASK;
 
 	force_pt_level = mapping_level_dirty_bitmap(vcpu, gfn);
 	if (likely(!force_pt_level)) {

@@ -2707,6 +2811,9 @@ static int nonpaging_map(struct kvm_vcpu *vcpu, gva_t v, int write, gfn_t gfn,
 	} else
 		level = PT_PAGE_TABLE_LEVEL;
 
+	if (fast_page_fault(vcpu, v, level, error_code))
+		return 0;
+
 	mmu_seq = vcpu->kvm->mmu_notifier_seq;
 	smp_rmb();
 

@@ -3095,7 +3202,7 @@ static int nonpaging_page_fault(struct kvm_vcpu *vcpu, gva_t gva,
 	gfn = gva >> PAGE_SHIFT;
 
 	return nonpaging_map(vcpu, gva & PAGE_MASK,
-			     error_code & PFERR_WRITE_MASK, gfn, prefault);
+			     error_code, gfn, prefault);
 }
 
 static int kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gva_t gva, gfn_t gfn)

@@ -3175,6 +3282,9 @@ static int tdp_page_fault(struct kvm_vcpu *vcpu, gva_t gpa, u32 error_code,
 	} else
 		level = PT_PAGE_TABLE_LEVEL;
 
+	if (fast_page_fault(vcpu, gpa, level, error_code))
+		return 0;
+
 	mmu_seq = vcpu->kvm->mmu_notifier_seq;
 	smp_rmb();
Signature · 1 line
1.7.7.6
author
#16 lore raw

PATCH v7 8/10 KVM: MMU: trace fast page fault

In reply to Xiao Guangrong

To see what happen on this path and help us to optimize it

Signed-off-by Xiao Guangrong <[email protected]>
---
 arch/x86/kvm/mmu.c      |    2 ++
 arch/x86/kvm/mmutrace.h |   38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 0 deletions(-)
arch/x86/kvm/mmu.c +2 -0
index 37c0230..fc221f9 100644
@@ -2778,6 +2778,8 @@ static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level,
 	 */
 	ret = fast_pf_fix_direct_spte(vcpu, iterator.sptep, spte);
 exit:
+	trace_fast_page_fault(vcpu, gva, error_code, iterator.sptep,
+			      spte, ret);
 	walk_shadow_page_lockless_end(vcpu);
 
 	return ret;
arch/x86/kvm/mmutrace.h +38 -0
index 89fb0e8..c364abc 100644
@@ -243,6 +243,44 @@ TRACE_EVENT(
 	TP_printk("addr:%llx gfn %llx access %x", __entry->addr, __entry->gfn,
 		  __entry->access)
 );
+
+#define __spte_satisfied(__spte)				\
+	(__entry->retry && is_writable_pte(__entry->__spte))
+
+TRACE_EVENT(
+	fast_page_fault,
+	TP_PROTO(struct kvm_vcpu *vcpu, gva_t gva, u32 error_code,
+		 u64 *sptep, u64 old_spte, bool retry),
+	TP_ARGS(vcpu, gva, error_code, sptep, old_spte, retry),
+
+	TP_STRUCT__entry(
+		__field(int, vcpu_id)
+		__field(gva_t, gva)
+		__field(u32, error_code)
+		__field(u64 *, sptep)
+		__field(u64, old_spte)
+		__field(u64, new_spte)
+		__field(bool, retry)
+	),
+
+	TP_fast_assign(
+		__entry->vcpu_id = vcpu->vcpu_id;
+		__entry->gva = gva;
+		__entry->error_code = error_code;
+		__entry->sptep = sptep;
+		__entry->old_spte = old_spte;
+		__entry->new_spte = *sptep;
+		__entry->retry = retry;
+	),
+
+	TP_printk("vcpu %d gva %lx error_code %s sptep %p old %#llx"
+		  " new %llx spurious %d fixed %d", __entry->vcpu_id,
+		  __entry->gva, __print_flags(__entry->error_code, "|",
+		  kvm_mmu_trace_pferr_flags), __entry->sptep,
+		  __entry->old_spte, __entry->new_spte,
+		  __spte_satisfied(old_spte), __spte_satisfied(new_spte)
+	)
+);
 #endif /* _TRACE_KVMMMU_H */
 
 #undef TRACE_INCLUDE_PATH
Signature · 1 line
1.7.7.6
author
#17 lore raw

PATCH v7 9/10 KVM: MMU: fix kvm_mmu_pagetable_walk tracepoint

In reply to Xiao Guangrong

The P bit of page fault error code is missed in this tracepoint, fix it by passing the full error code

Signed-off-by Xiao Guangrong <[email protected]>
---
 arch/x86/kvm/mmutrace.h    |    7 +++----
 arch/x86/kvm/paging_tmpl.h |    3 +--
 2 files changed, 4 insertions(+), 6 deletions(-)
arch/x86/kvm/mmutrace.h +3 -4
index c364abc..cd6e983 100644
@@ -54,8 +54,8 @@
  */
 TRACE_EVENT(
 	kvm_mmu_pagetable_walk,
-	TP_PROTO(u64 addr, int write_fault, int user_fault, int fetch_fault),
-	TP_ARGS(addr, write_fault, user_fault, fetch_fault),
+	TP_PROTO(u64 addr, u32 pferr),
+	TP_ARGS(addr, pferr),
 
 	TP_STRUCT__entry(
 		__field(__u64, addr)

@@ -64,8 +64,7 @@ TRACE_EVENT(
 
 	TP_fast_assign(
 		__entry->addr = addr;
-		__entry->pferr = (!!write_fault << 1) | (!!user_fault << 2)
-		                 | (!!fetch_fault << 4);
+		__entry->pferr = pferr;
 	),
 
 	TP_printk("addr %llx pferr %x %s", __entry->addr, __entry->pferr,
arch/x86/kvm/paging_tmpl.h +1 -2
index 34f9709..bb7cf01 100644
@@ -154,8 +154,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
 	const int fetch_fault = access & PFERR_FETCH_MASK;
 	u16 errcode = 0;
 
-	trace_kvm_mmu_pagetable_walk(addr, write_fault, user_fault,
-				     fetch_fault);
+	trace_kvm_mmu_pagetable_walk(addr, access);
 retry_walk:
 	eperm = false;
 	walker->level = mmu->root_level;
Signature · 1 line
1.7.7.6
author
#18 lore raw

PATCH v7 10/10 KVM: MMU: document mmu-lock and fast page fault

In reply to Xiao Guangrong

Document fast page fault and mmu-lock in locking.txt

Signed-off-by Xiao Guangrong <[email protected]>
---
 Documentation/virtual/kvm/locking.txt |  130 ++++++++++++++++++++++++++++++++-
 1 files changed, 129 insertions(+), 1 deletions(-)
Documentation/virtual/kvm/locking.txt +129 -1
index 3b4cd3b..41b7ac9 100644
@@ -6,7 +6,129 @@ KVM Lock Overview
 
 (to be written)
 
-2. Reference
+2: Exception
+------------
+
+Fast page fault:
+
+Fast page fault is the fast path which fixes the guest page fault out of
+the mmu-lock on x86. Currently, the page fault can be fast only if the
+shadow page table is present and it is caused by write-protect, that means
+we just need change the W bit of the spte.
+
+What we use to avoid all the race is the SPTE_HOST_WRITEABLE bit and
+SPTE_MMU_WRITEABLE bit on the spte:
+- SPTE_HOST_WRITEABLE means the gfn is writable on host.
+- SPTE_MMU_WRITEABLE means the gfn is writable on mmu. The bit is set when
+  the gfn is writable on guest mmu and it is not write-protected by shadow
+  page write-protection.
+
+On fast page fault path, we will use cmpxchg to atomically set the spte W
+bit if spte.SPTE_HOST_WRITEABLE = 1 and spte.SPTE_WRITE_PROTECT = 1, this
+is safe because whenever changing these bits can be detected by cmpxchg.
+
+But we need carefully check these cases:
+1): The mapping from gfn to pfn
+The mapping from gfn to pfn may be changed since we can only ensure the pfn
+is not changed during cmpxchg. This is a ABA problem, for example, below case
+will happen:
+
+At the beginning:
+gpte = gfn1
+gfn1 is mapped to pfn1 on host
+spte is the shadow page table entry corresponding with gpte and
+spte = pfn1
+
+   VCPU 0                           VCPU0
+on fast page fault path:
+
+   old_spte = *spte;
+                                 pfn1 is swapped out:
+                                    spte = 0;
+
+                                 pfn1 is re-alloced for gfn2.
+
+                                 gpte is changed to point to
+                                 gfn2 by the guest:
+                                    spte = pfn1;
+
+   if (cmpxchg(spte, old_spte, old_spte+W)
+	mark_page_dirty(vcpu->kvm, gfn1)
+             OOPS!!!
+
+We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap.
+
+For direct sp, we can easily avoid it since the spte of direct sp is fixed
+to gfn. For indirect sp, before we do cmpxchg, we call gfn_to_pfn_atomic()
+to pin gfn to pfn, because after gfn_to_pfn_atomic():
+- We have held the refcount of pfn that means the pfn can not be freed and
+  be reused for another gfn.
+- The pfn is writable that means it can not be shared between different gfns
+  by KSM.
+
+Then, we can ensure the dirty bitmaps is correctly set for a gfn.
+
+Currently, to simplify the whole things, we disable fast page fault for
+indirect shadow page.
+
+2): Dirty bit tracking
+In the origin code, the spte can be fast updated (non-atomically) if the
+spte is read-only and the Accessed bit has already been set since the
+Accessed bit and Dirty bit can not be lost.
+
+But it is not true after fast page fault since the spte can be marked
+writable between reading spte and updating spte. Like below case:
+
+At the beginning:
+spte.W = 0
+spte.Accessed = 1
+
+   VCPU 0                                       VCPU0
+In mmu_spte_clear_track_bits():
+
+   old_spte = *spte;
+
+   /* 'if' condition is satisfied. */
+   if (old_spte.Accssed == 1 &&
+        old_spte.W == 0)
+      spte = 0ull;
+                                         on fast page fault path:
+                                             spte.W = 1
+                                         memory write on the spte:
+                                             spte.Dirty = 1
+
+
+   else
+      old_spte = xchg(spte, 0ull)
+
+
+   if (old_spte.Accssed == 1)
+      kvm_set_pfn_accessed(spte.pfn);
+   if (old_spte.Dirty == 1)
+      kvm_set_pfn_dirty(spte.pfn);
+      OOPS!!!
+
+The Dirty bit is lost in this case.
+
+In order to avoid this kind of issue, we always treat the spte as "volatile"
+if it can be updated out of mmu-lock, see spte_has_volatile_bits(), it means,
+the spte is always atomicly updated in this case.
+
+3): flush tlbs due to spte updated
+If the spte is updated from writable to readonly, we should flush all TLBs,
+otherwise rmap_write_protect will find a read-only spte, even though the
+writable spte might be cached on a CPU's TLB.
+
+As mentioned before, the spte can be updated to writable out of mmu-lock on
+fast page fault path, in order to easily audit the path, we see if TLBs need
+be flushed caused by this reason in mmu_spte_update() since this is a common
+function to update spte (present -> present).
+
+Since the spte is "volatile" if it can be updated out of mmu-lock, we always
+atomicly update the spte, the race caused by fast page fault can be avoided,
+See the comments in spte_has_volatile_bits() and mmu_spte_update().
+
+3. Reference
 ------------
 
 Name:		kvm_lock

@@ -23,3 +145,9 @@ Arch:		x86
 Protects:	- kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
 		- tsc offset in vmcb
 Comment:	'raw' because updating the tsc offsets must not be preempted.
+
+Name:		kvm->mmu_lock
+Type:		spinlock_t
+Arch:		any
+Protects:	-shadow page/shadow tlb entry
+Comment:	it is a spinlock since it is used in mmu notifier.
Signature · 1 line
1.7.7.6
#19 lore raw
In reply to Xiao Guangrong
Quoted 10 lines · Xiao Guangrong

Changlog:
- always atomicly update the spte if it can be updated out of mmu-lock
- rename spte_can_be_writable() to spte_is_locklessly_modifiable()
- cleanup and comment spte_write_protect()

Performance result:
(The benchmark can be found at: http://www.spinics.net/lists/kvm/msg73011.html)

before after
Run 10 times, Avg time: 538233957 ns. 249809853 ns. +53.6%

Looks fine to me.

#20 lore raw
In reply to Xiao Guangrong
Quoted 11 lines · Xiao Guangrong

Changlog:
- always atomicly update the spte if it can be updated out of mmu-lock
- rename spte_can_be_writable() to spte_is_locklessly_modifiable()
- cleanup and comment spte_write_protect()

Performance result:
(The benchmark can be found at: http://www.spinics.net/lists/kvm/msg73011.html)

before after
Run 10 times, Avg time: 538233957 ns. 249809853 ns. +53.6%

Thanks, all applied. Thanks for your patience while we reviewed all the iterations of this patchset.

Signature · 1 line
error compiling committee.c: too many arguments to function