KVM: SVM: delete avic_vm_id_bitmap (2 megabyte static array)

With lightly tweaked defconfig:

text data bss dec hex filename
11259661 5109408 2981888 19350957 12745ad vmlinux.before 11259661 5109408 884736 17253805 10745ad vmlinux.after

Only compile-tested.

Signed-off-by Denys Vlasenko <[email protected]>

Cc Joerg Roedel <[email protected]>

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

---

2017-08-11 22:11+0200, Denys Vlasenko:

Quoted 18 lines

With lightly tweaked defconfig:

text data bss dec hex filename
11259661 5109408 2981888 19350957 12745ad vmlinux.before 11259661 5109408 884736 17253805 10745ad vmlinux.after

Only compile-tested.

Signed-off-by Denys Vlasenko <[email protected]>

Cc Joerg Roedel <[email protected]>

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Quoted 1 line

---

Ah, I suspected my todo wasn't this short; thanks for the patch!

arch/x86/kvm/svm.c static int avic_vm_init(struct kvm *kvm) · line 1438

Quoted 7 lines

arch/x86/kvm/svm.c +3 -0

@@ -1468,6 +1433,22 @@ static int avic_vm_init(struct kvm *kvm)
 	clear_page(page_address(l_page));
 
 	spin_lock_irqsave(&svm_vm_data_hash_lock, flags);
+ again:
+	vm_id = next_vm_id = (next_vm_id + 1) & AVIC_VM_ID_MASK;
+	if (vm_id == 0) { /* id is 1-based, zero is not okay */

Suravee, did the reserved zero mean something?

Review static int avic_vm_init(struct kvm *kvm) · line 1448

Quoted 10 lines

(quoted diff) +10 -0

+		next_vm_id_wrapped = 1;
+		goto again;
+	}
+	/* Is it still in use? Only possible if wrapped at least once */
+	if (next_vm_id_wrapped) {
+		hash_for_each_possible(svm_vm_data_hash, ka, hnode, vm_id) {
+			struct kvm *k2 = container_of(ka, struct kvm, arch);
+			struct kvm_arch *vd2 = &k2->arch;
+			if (vd2->avic_vm_id == vm_id)
+				goto again;

Although hitting the case where all 2^24 ids are already used is practically impossible, I don't like the loose end ...

What about applying something like the following on top? ---8<---
Subject: [PATCH] KVM: SVM: refactor avic VM ID allocation

We missed protection in case someone deserving a medal allocated 2^24 VMs and got a deadlock instead. I think it is nicer without the goto even if we droppped the error handling.

Signed-off-by Radim Krčmář <[email protected]>

---
 arch/x86/kvm/svm.c | 58 +++++++++++++++++++++++++++++++++++-------------------
 1 file changed, 38 insertions(+), 20 deletions(-)

arch/x86/kvm/svm.c +38 -20

index c21b49b5ee96..4d9ee8d76db3 100644
@@ -985,8 +985,6 @@ static void disable_nmi_singlestep(struct vcpu_svm *svm)
  */
 #define SVM_VM_DATA_HASH_BITS	8
 static DEFINE_HASHTABLE(svm_vm_data_hash, SVM_VM_DATA_HASH_BITS);
-static u32 next_vm_id = 0;
-static bool next_vm_id_wrapped = 0;
 static DEFINE_SPINLOCK(svm_vm_data_hash_lock);
 
 /* Note:

@@ -1385,6 +1383,38 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
+static bool avic_vm_id_is_used(u32 vm_id)
+{
+	struct kvm_arch *ka;
+
+	hash_for_each_possible(svm_vm_data_hash, ka, hnode, vm_id)
+		if (ka->avic_vm_id == vm_id)
+			return true;
+
+	return false;
+}
+
+static u32 avic_get_next_vm_id(void)
+{
+	static u32 next_vm_id = 0;
+	static bool next_vm_id_wrapped = false;
+	unsigned i;
+
+	for (i = 0; i < AVIC_VM_ID_NR; i++) {
+		next_vm_id = (next_vm_id + 1) & AVIC_VM_ID_MASK;
+
+		if (next_vm_id == 0) { /* id is 1-based, zero is not okay */
+			next_vm_id = 1;
+			next_vm_id_wrapped = true;
+		}
+
+		if (!next_vm_id_wrapped || !avic_vm_id_is_used(next_vm_id))
+			return next_vm_id;
+	}
+
+	return 0;
+}
+
 static void avic_vm_destroy(struct kvm *kvm)
 {
 	unsigned long flags;

@@ -1410,8 +1440,6 @@ static int avic_vm_init(struct kvm *kvm)
 	struct kvm_arch *vm_data = &kvm->arch;
 	struct page *p_page;
 	struct page *l_page;
-	struct kvm_arch *ka;
-	u32 vm_id;
 
 	if (!avic)
 		return 0;

@@ -1432,28 +1460,18 @@ static int avic_vm_init(struct kvm *kvm)
 	vm_data->avic_logical_id_table_page = l_page;
 	clear_page(page_address(l_page));
 
+	err = -EAGAIN;
 	spin_lock_irqsave(&svm_vm_data_hash_lock, flags);
- again:
-	vm_id = next_vm_id = (next_vm_id + 1) & AVIC_VM_ID_MASK;
-	if (vm_id == 0) { /* id is 1-based, zero is not okay */
-		next_vm_id_wrapped = 1;
-		goto again;
-	}
-	/* Is it still in use? Only possible if wrapped at least once */
-	if (next_vm_id_wrapped) {
-		hash_for_each_possible(svm_vm_data_hash, ka, hnode, vm_id) {
-			struct kvm *k2 = container_of(ka, struct kvm, arch);
-			struct kvm_arch *vd2 = &k2->arch;
-			if (vd2->avic_vm_id == vm_id)
-				goto again;
-		}
-	}
-	vm_data->avic_vm_id = vm_id;
+	vm_data->avic_vm_id = avic_get_next_vm_id();
+	if (!vm_data->avic_vm_id)
+		goto unlock;
 	hash_add(svm_vm_data_hash, &vm_data->hnode, vm_data->avic_vm_id);
 	spin_unlock_irqrestore(&svm_vm_data_hash_lock, flags);
 
 	return 0;
 
+unlock:
+	spin_unlock_irqrestore(&svm_vm_data_hash_lock, flags);
 free_avic:
 	avic_vm_destroy(kvm);
 	return err;

Quoted 33 lines · Radim Krčmář

2017-08-11 22:11+0200, Denys Vlasenko:

Quoted 18 lines

With lightly tweaked defconfig:

text data bss dec hex filename
11259661 5109408 2981888 19350957 12745ad vmlinux.before 11259661 5109408 884736 17253805 10745ad vmlinux.after

Only compile-tested.

Signed-off-by Denys Vlasenko <[email protected]>

Cc Joerg Roedel <[email protected]>

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Quoted 1 line

---

Ah, I suspected my todo wasn't this short; thanks for the patch!

arch/x86/kvm/svm.c static int avic_vm_init(struct kvm *kvm) · line 1438

Quoted 7 lines

arch/x86/kvm/svm.c +3 -0

@@ -1468,6 +1433,22 @@ static int avic_vm_init(struct kvm *kvm)
 	clear_page(page_address(l_page));
 
 	spin_lock_irqsave(&svm_vm_data_hash_lock, flags);
+ again:
+	vm_id = next_vm_id = (next_vm_id + 1) & AVIC_VM_ID_MASK;
+	if (vm_id == 0) { /* id is 1-based, zero is not okay */

Suravee, did the reserved zero mean something?

Quoted 13 lines

Review static int avic_vm_init(struct kvm *kvm) · line 1448

Quoted 10 lines

(quoted diff) +10 -0

+		next_vm_id_wrapped = 1;
+		goto again;
+	}
+	/* Is it still in use? Only possible if wrapped at least once */
+	if (next_vm_id_wrapped) {
+		hash_for_each_possible(svm_vm_data_hash, ka, hnode, vm_id) {
+			struct kvm *k2 = container_of(ka, struct kvm, arch);
+			struct kvm_arch *vd2 = &k2->arch;
+			if (vd2->avic_vm_id == vm_id)
+				goto again;

Although hitting the case where all 2^24 ids are already used is practically impossible, I don't like the loose end ...

I think even the case where 2^16 ids are used deserves a medal. Why don't we just make the bitmap 8 KiB and call it a day? :)

Quoted 45 lines · Paolo Bonzini

Quoted 44 lines · Radim Krčmář

2017-08-11 22:11+0200, Denys Vlasenko:

Quoted 18 lines

With lightly tweaked defconfig:

text data bss dec hex filename
11259661 5109408 2981888 19350957 12745ad vmlinux.before 11259661 5109408 884736 17253805 10745ad vmlinux.after

Only compile-tested.

Signed-off-by Denys Vlasenko <[email protected]>

Cc Joerg Roedel <[email protected]>

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Cc [email protected]

Quoted 1 line

---

Ah, I suspected my todo wasn't this short; thanks for the patch!

arch/x86/kvm/svm.c static int avic_vm_init(struct kvm *kvm) · line 1438

Quoted 7 lines

arch/x86/kvm/svm.c +3 -0

@@ -1468,6 +1433,22 @@ static int avic_vm_init(struct kvm *kvm)
     clear_page(page_address(l_page));
 
     spin_lock_irqsave(&svm_vm_data_hash_lock, flags);
+ again:
+    vm_id = next_vm_id = (next_vm_id + 1) & AVIC_VM_ID_MASK;
+    if (vm_id == 0) { /* id is 1-based, zero is not okay */

Suravee, did the reserved zero mean something?

Quoted 10 lines

(quoted diff) +10 -0

+        next_vm_id_wrapped = 1;
+        goto again;
+    }
+    /* Is it still in use? Only possible if wrapped at least once */
+    if (next_vm_id_wrapped) {
+        hash_for_each_possible(svm_vm_data_hash, ka, hnode, vm_id) {
+            struct kvm *k2 = container_of(ka, struct kvm, arch);
+            struct kvm_arch *vd2 = &k2->arch;
+            if (vd2->avic_vm_id == vm_id)
+                goto again;

Quoted 5 lines

Quoted 2 lines

Although hitting the case where all 2^24 ids are already used is practically impossible, I don't like the loose end ...

I think even the case where 2^16 ids are used deserves a medal. Why don't we just make the bitmap 8 KiB and call it a day? :)

Well, the RAM is cheap, but a 4-byte variable is still thousands of times smaller than a 8K bitmap.

Since a 256 element hash table is used here, you need to have ~one hundred VMs to start seeing (very small) degradation in speed of creation of new VMs compared to bitmap approach, and that is only after 16777216 VMs were created since reboot.

If you want to spend RAM on speeding this up, you can increase hash table size
instead. That would speed up avic_ga_log_notifier() too.