kvm: host-side changes for tmem on KVM

From: Akshay Karle <[email protected]>
Subject: [RFC 1/2] kvm: host-side changes for tmem on KVM

Working at host:
Once the guest exits to the kvm host, the host determines that the guest exited to perform some tmem operation(done at kvm_emulate_hypercall)and then we use zcache to implement this required operations(performed by kvm_pv_tmem_op).

---
Diffstat for host patch:
 arch/x86/include/asm/kvm_host.h      |    1
 arch/x86/kvm/x86.c                   |    4 + drivers/staging/zcache/zcache-main.c |   98 ++++++++++++++++++++++++++++++++---

 3 files changed, 95 insertions(+), 8 deletions(-)

diff -Napur vanilla/linux-3.1.5/arch/x86/include/asm/kvm_host.h linux-3.1.5//arch/x86/include/asm/kvm_host.h

/arch/x86/include/asm/kvm_host.h        2012-03-05 14:09:41.648006153 +0530 renamed from linux-3.1.5/arch/x86/include/asm/kvm_host.h 2011-12-09 22:27:05.000000000 +0530 +1 -0

@@ -668,6 +668,7 @@ int emulator_write_phys(struct kvm_vcpu
                          const void *val, int bytes);
  int kvm_pv_mmu_op(struct kvm_vcpu *vcpu, unsigned long bytes,
                  gpa_t addr, unsigned long *ret);
+int kvm_pv_tmem_op(struct kvm_vcpu *vcpu, gpa_t addr, unsigned long *ret);
  u8 kvm_get_guest_memory_type(struct kvm_vcpu *vcpu, gfn_t gfn);
 
  extern bool tdp_enabled;

diff -Napur vanilla/linux-3.1.5/arch/x86/kvm/x86.c linux-3.1.5//arch/x86/kvm/x86.c

/arch/x86/kvm/x86.c     2012-03-05 14:09:41.652006083 +0530 renamed from linux-3.1.5/arch/x86/kvm/x86.c      2011-12-09 22:27:05.000000000 +0530 +0 -0

@@ -5267,6 +5267,10 @@ int kvm_emulate_hypercall(struct kvm_vcp

       case KVM_HC_MMU_OP:
               r = kvm_pv_mmu_op(vcpu, a0, hc_gpa(vcpu, a1, a2), &ret);
               break;
+       case KVM_HC_TMEM:
+               r = kvm_pv_tmem_op(vcpu, a0, &ret);
+               ret = ret - 1000;
+               break;
       default:
               ret = -KVM_ENOSYS;
               break;
diff -Napur vanilla/linux-3.1.5/drivers/staging/zcache/zcache-main.c linux-3.1.5//drivers/staging/zcache/zcache-main.c

/drivers/staging/zcache/zcache-main.c   2012-03-05 14:10:31.264006031 +0530 renamed from linux-3.1.5/drivers/staging/zcache/zcache-main.c    2011-12-09 22:27:05.000000000 +0530 +0 -2

  #include "../zram/xvmalloc.h" /* if built in drivers/staging */
 

@@ -669,7 +670,6 @@ static struct zv_hdr *zv_create(struct x
        int chunks = (alloc_size + (CHUNK_SIZE - 1)) >> CHUNK_SHIFT;
        int ret;
 
-       BUG_ON(!irqs_disabled());
        BUG_ON(chunks >= NCHUNKS);
        ret = xv_malloc(xvpool, alloc_size,
                        &page, &offset, ZCACHE_GFP_MASK);

@@ -1313,7 +1313,6 @@ static int zcache_compress(struct page *
        unsigned char *wmem = __get_cpu_var(zcache_workmem);
        char *from_va;
 
-       BUG_ON(!irqs_disabled());
        if (unlikely(dmem == NULL || wmem == NULL))

               goto out;  /* no buffer, so can't compress */ from_va = kmap_atomic(from, KM_USER0);

/drivers/staging/zcache/zcache-main.c   2012-03-05 14:10:31.264006031 +0530 renamed from linux-3.1.5/drivers/staging/zcache/zcache-main.c    2011-12-09 22:27:05.000000000 +0530 +89 -7

@@ -1533,7 +1532,6 @@ static int zcache_put_page(int cli_id, i
        struct tmem_pool *pool;
        int ret = -1;
 
-       BUG_ON(!irqs_disabled());
        pool = zcache_get_pool_by_id(cli_id, pool_id);
        if (unlikely(pool == NULL))
                goto out;

@@ -1898,6 +1896,67 @@ struct frontswap_ops zcache_frontswap_re
  #endif
 
  /*
+ * tmem op to support tmem in kvm guests
+ */
+
+int kvm_pv_tmem_op(struct kvm_vcpu *vcpu, gpa_t addr, unsigned long *ret)
+{
+       struct tmem_ops op;
+       struct tmem_oid oid;
+       uint64_t pfn;
+       struct page *page;
+       int r;
+
+       r = kvm_read_guest(vcpu->kvm, addr, &op, sizeof(op));
+       if (r < 0)
+               return r;
+
+       switch (op.cmd) {
+       case TMEM_NEW_POOL:
+               *ret = zcache_new_pool(op.u.new.cli_id, op.u.new.flags);
+               break;
+       case TMEM_DESTROY_POOL:
+               *ret = zcache_destroy_pool(op.u.gen.cli_id, op.pool_id);
+               break;
+       case TMEM_NEW_PAGE:
+               break;
+       case TMEM_PUT_PAGE:
+               pfn = gfn_to_pfn(vcpu->kvm, op.u.gen.pfn);
+               page = pfn_to_page(pfn);
+               oid.oid[0] = op.u.gen.oid[0];
+               oid.oid[1] = op.u.gen.oid[1];
+               oid.oid[2] = op.u.gen.oid[2];
+               VM_BUG_ON(!PageLocked(page));
+               *ret = zcache_put_page(op.u.gen.cli_id, op.pool_id,
+                               &oid, op.u.gen.index, page);
+               break;
+       case TMEM_GET_PAGE:
+               pfn = gfn_to_pfn(vcpu->kvm, op.u.gen.pfn);
+               page = pfn_to_page(pfn);
+               oid.oid[0] = op.u.gen.oid[0];
+               oid.oid[1] = op.u.gen.oid[1];
+               oid.oid[2] = op.u.gen.oid[2];
+               *ret = zcache_get_page(TMEM_CLI, op.pool_id,
+                               &oid, op.u.gen.index, page);
+               break;
+       case TMEM_FLUSH_PAGE:
+               oid.oid[0] = op.u.gen.oid[0];
+               oid.oid[1] = op.u.gen.oid[1];
+               oid.oid[2] = op.u.gen.oid[2];
+               *ret = zcache_flush_page(op.u.gen.cli_id, op.pool_id,
+                               &oid, op.u.gen.index);
+               break;
+       case TMEM_FLUSH_OBJECT:
+               oid.oid[0] = op.u.gen.oid[0];
+               oid.oid[1] = op.u.gen.oid[1];
+               oid.oid[2] = op.u.gen.oid[2];
+               *ret = zcache_flush_object(op.u.gen.cli_id, op.pool_id, &oid);
+               break;
+       }
+       return 0;
+}
+
+/*
  * zcache initialization
  * NOTE FOR NOW zcache MUST BE PROVIDED AS A KERNEL BOOT PARAMETER OR
  * NOTHING HAPPENS!

@@ -1934,10 +1993,19 @@ static int __init no_frontswap(char *s)
 
  __setup("nofrontswap", no_frontswap);
 
+static int kvm_tmem_enabled = 0;
+
+static int __init enable_kvm_tmem(char *s)
+{
+       kvm_tmem_enabled = 1;
+       return 1;
+}
+
+__setup("kvmtmem", enable_kvm_tmem);
+
  static int __init zcache_init(void)
  {
        int ret = 0;
-
  #ifdef CONFIG_SYSFS
        ret = sysfs_create_group(mm_kobj, &zcache_attr_group);
        if (ret) {

@@ -1946,7 +2014,7 @@ static int __init zcache_init(void)
        }
  #endif /* CONFIG_SYSFS */
  #if defined(CONFIG_CLEANCACHE) || defined(CONFIG_FRONTSWAP)
-       if (zcache_enabled) {
+       if (zcache_enabled || kvm_tmem_enabled) {
                unsigned int cpu;
 
                tmem_register_hostops(&zcache_hostops);

@@ -1966,11 +2034,25 @@ static int __init zcache_init(void)
                                sizeof(struct tmem_objnode), 0, 0, NULL);
        zcache_obj_cache = kmem_cache_create("zcache_obj",
                                sizeof(struct tmem_obj), 0, 0, NULL);
-       ret = zcache_new_client(LOCAL_CLIENT);
-       if (ret) {
-               pr_err("zcache: can't create client\n");
+       if(kvm_tmem_enabled) {
+               ret = zcache_new_client(TMEM_CLI);
+               if(ret) {
+                       pr_err("zcache: can't create client\n");
+                       goto out;
+               }
+               zbud_init();
+               register_shrinker(&zcache_shrinker);
+               pr_info("zcache: transcendent memory enabled using kernel "
+                       "for kvm guests\n");
                goto out;
        }
+       else {
+               ret = zcache_new_client(LOCAL_CLIENT);
+               if (ret) {
+                       pr_err("zcache: can't create client\n");
+                       goto out;
+               }
+       }
  #endif
  #ifdef CONFIG_CLEANCACHE
        if (zcache_enabled && use_cleancache) {
 
 
--

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From: Akshay Karle <[email protected]>
Subject: [RFC 1/2] kvm: host-side changes for tmem on KVM

Working at host:
Once the guest exits to the kvm host, the host determines that the guest exited to perform some tmem operation(done at kvm_emulate_hypercall)and then we use zcache to implement this required operations(performed by kvm_pv_tmem_op).

---
Diffstat for host patch:

 arch/x86/include/asm/kvm_host.h      |    1 
 arch/x86/kvm/x86.c                   |    4 +
 drivers/staging/zcache/zcache-main.c |   98 ++++++++++++++++++++++++++++++++---
 3 files changed, 95 insertions(+), 8 deletions(-)

diff -Napur vanilla/linux-3.1.5/arch/x86/include/asm/kvm_host.h linux-3.1.5//arch/x86/include/asm/kvm_host.h

/arch/x86/include/asm/kvm_host.h renamed from linux-3.1.5/arch/x86/include/asm/kvm_host.h +1 -0

@@ -668,6 +668,7 @@ int emulator_write_phys(struct kvm_vcpu
 			  const void *val, int bytes);
 int kvm_pv_mmu_op(struct kvm_vcpu *vcpu, unsigned long bytes,
 		  gpa_t addr, unsigned long *ret);
+int kvm_pv_tmem_op(struct kvm_vcpu *vcpu, gpa_t addr, unsigned long *ret);
 u8 kvm_get_guest_memory_type(struct kvm_vcpu *vcpu, gfn_t gfn);
 
 extern bool tdp_enabled;

diff -Napur vanilla/linux-3.1.5/arch/x86/kvm/x86.c linux-3.1.5//arch/x86/kvm/x86.c

/arch/x86/kvm/x86.c renamed from linux-3.1.5/arch/x86/kvm/x86.c +1 -0

+		break;
 	default:
 		ret = -KVM_ENOSYS;
 		break;

diff -Napur vanilla/linux-3.1.5/drivers/staging/zcache/zcache-main.c linux-3.1.5//drivers/staging/zcache/zcache-main.c

/drivers/staging/zcache/zcache-main.c renamed from linux-3.1.5/drivers/staging/zcache/zcache-main.c +1 -0

@@ -30,6 +30,7 @@
 #include <linux/atomic.h>
 #include <linux/math64.h>
 #include "tmem.h"
+#include "kvm-tmem.h"
 
 #include "../zram/xvmalloc.h" /* if built in drivers/staging */
 

/drivers/staging/zcache/zcache-main.c renamed from linux-3.1.5/drivers/staging/zcache/zcache-main.c +61 -2

 	BUG_ON(chunks >= NCHUNKS);
 	ret = xv_malloc(xvpool, alloc_size,
 			&page, &offset, ZCACHE_GFP_MASK);

@@ -1313,7 +1313,6 @@ static int zcache_compress(struct page *
 	unsigned char *wmem = __get_cpu_var(zcache_workmem);
 	char *from_va;
 
-	BUG_ON(!irqs_disabled());
 	if (unlikely(dmem == NULL || wmem == NULL))
 		goto out;  /* no buffer, so can't compress */
 	from_va = kmap_atomic(from, KM_USER0);

@@ -1533,7 +1532,6 @@ static int zcache_put_page(int cli_id, i
 	struct tmem_pool *pool;
 	int ret = -1;
 
-	BUG_ON(!irqs_disabled());
 	pool = zcache_get_pool_by_id(cli_id, pool_id);
 	if (unlikely(pool == NULL))
 		goto out;

@@ -1898,6 +1896,67 @@ struct frontswap_ops zcache_frontswap_re
 #endif
 
 /*
+ * tmem op to support tmem in kvm guests
+ */
+
+int kvm_pv_tmem_op(struct kvm_vcpu *vcpu, gpa_t addr, unsigned long *ret)
+{
+	struct tmem_ops op;
+	struct tmem_oid oid;
+	uint64_t pfn;
+	struct page *page;
+	int r;
+
+	r = kvm_read_guest(vcpu->kvm, addr, &op, sizeof(op));
+	if (r < 0)
+		return r;
+
+	switch (op.cmd) {
+	case TMEM_NEW_POOL:
+		*ret = zcache_new_pool(op.u.new.cli_id, op.u.new.flags);
+		break;
+	case TMEM_DESTROY_POOL:
+		*ret = zcache_destroy_pool(op.u.gen.cli_id, op.pool_id);
+		break;
+	case TMEM_NEW_PAGE:
+		break;
+	case TMEM_PUT_PAGE:
+		pfn = gfn_to_pfn(vcpu->kvm, op.u.gen.pfn);
+		page = pfn_to_page(pfn);
+		oid.oid[0] = op.u.gen.oid[0];
+		oid.oid[1] = op.u.gen.oid[1];
+		oid.oid[2] = op.u.gen.oid[2];
+		VM_BUG_ON(!PageLocked(page));
+		*ret = zcache_put_page(op.u.gen.cli_id, op.pool_id,
+				&oid, op.u.gen.index, page);
+		break;
+	case TMEM_GET_PAGE:
+		pfn = gfn_to_pfn(vcpu->kvm, op.u.gen.pfn);
+		page = pfn_to_page(pfn);
+		oid.oid[0] = op.u.gen.oid[0];
+		oid.oid[1] = op.u.gen.oid[1];
+		oid.oid[2] = op.u.gen.oid[2];
+		*ret = zcache_get_page(TMEM_CLI, op.pool_id,
+				&oid, op.u.gen.index, page);
+		break;
+	case TMEM_FLUSH_PAGE:
+		oid.oid[0] = op.u.gen.oid[0];
+		oid.oid[1] = op.u.gen.oid[1];
+		oid.oid[2] = op.u.gen.oid[2];
+		*ret = zcache_flush_page(op.u.gen.cli_id, op.pool_id,
+				&oid, op.u.gen.index);
+		break;
+	case TMEM_FLUSH_OBJECT:
+		oid.oid[0] = op.u.gen.oid[0];
+		oid.oid[1] = op.u.gen.oid[1];
+		oid.oid[2] = op.u.gen.oid[2];
+		*ret = zcache_flush_object(op.u.gen.cli_id, op.pool_id, &oid);
+		break;
+	}
+	return 0;
+}
+
+/*
  * zcache initialization
  * NOTE FOR NOW zcache MUST BE PROVIDED AS A KERNEL BOOT PARAMETER OR
  * NOTHING HAPPENS!

/drivers/staging/zcache/zcache-main.c renamed from linux-3.1.5/drivers/staging/zcache/zcache-main.c +10 -2

+
+static int __init enable_kvm_tmem(char *s)
+{
+	kvm_tmem_enabled = 1;
+	return 1;
+}
+
+__setup("kvmtmem", enable_kvm_tmem);
+
 static int __init zcache_init(void)
 {
 	int ret = 0;
-
 #ifdef CONFIG_SYSFS
 	ret = sysfs_create_group(mm_kobj, &zcache_attr_group);
 	if (ret) {

@@ -1946,7 +2014,7 @@ static int __init zcache_init(void)
 	}
 #endif /* CONFIG_SYSFS */
 #if defined(CONFIG_CLEANCACHE) || defined(CONFIG_FRONTSWAP)
-	if (zcache_enabled) {
+	if (zcache_enabled || kvm_tmem_enabled) {
 		unsigned int cpu;
 
 		tmem_register_hostops(&zcache_hostops);

/drivers/staging/zcache/zcache-main.c renamed from linux-3.1.5/drivers/staging/zcache/zcache-main.c +16 -0

+		ret = zcache_new_client(TMEM_CLI);
+		if(ret) {
+			pr_err("zcache: can't create client\n");
+			goto out;
+		}
+		zbud_init();
+		register_shrinker(&zcache_shrinker);
+		pr_info("zcache: transcendent memory enabled using kernel "
+			"for kvm guests\n");
 		goto out;
 	}
+	else {
+		ret = zcache_new_client(LOCAL_CLIENT);
+		if (ret) {
+			pr_err("zcache: can't create client\n");
+			goto out;
+		}
+	}
 #endif
 #ifdef CONFIG_CLEANCACHE
 	if (zcache_enabled && use_cleancache) {

Quoted 4 lines

Working at host:
Once the guest exits to the kvm host, the host determines that the guest exited to perform some tmem operation(done at kvm_emulate_hypercall)and then we use zcache to implement this required operations(performed by kvm_pv_tmem_op).

Do you need any modifications to the Kconfig file to reflect the KVM dependency?

Quoted 4 lines

diff -Napur vanilla/linux-3.1.5/arch/x86/kvm/x86.c linux-3.1.5//arch/x86/kvm/x86.c

/arch/x86/kvm/x86.c renamed from linux-3.1.5/arch/x86/kvm/x86.c +0 -0

@@ -5267,6 +5267,10 @@ int kvm_emulate_hypercall(struct kvm_vcp

/arch/x86/kvm/x86.c int kvm_emulate_hypercall(struct kvm_vcp · line 5272

Quoted 6 lines

/arch/x86/kvm/x86.c renamed from linux-3.1.5/arch/x86/kvm/x86.c +3 -0

 	case KVM_HC_MMU_OP:
 		r = kvm_pv_mmu_op(vcpu, a0, hc_gpa(vcpu, a1, a2), &ret);
 		break;
+	case KVM_HC_TMEM:
+		r = kvm_pv_tmem_op(vcpu, a0, &ret);
+		ret = ret - 1000;

That is rather odd. Why the subtraction of 1000?

/arch/x86/kvm/x86.c static struct zv_hdr *zv_create(struct x · line 672

Quoted 5 lines

/arch/x86/kvm/x86.c renamed from linux-3.1.5/arch/x86/kvm/x86.c +0 -1

@@ -669,7 +670,6 @@ static struct zv_hdr *zv_create(struct x
      int chunks = (alloc_size + (CHUNK_SIZE - 1)) >> CHUNK_SHIFT;
      int ret;
 
-     BUG_ON(!irqs_disabled());

Can you explain why?

/arch/x86/kvm/x86.c static int __init no_frontswap(char *s) · line 1996

Quoted 5 lines

/arch/x86/kvm/x86.c renamed from linux-3.1.5/arch/x86/kvm/x86.c +1 -0

@@ -1934,10 +1993,19 @@ static int __init no_frontswap(char *s)
 
 __setup("nofrontswap", no_frontswap);
 
+static int kvm_tmem_enabled = 0;

No need to declare it zero. Don't we want to have it running by default? So the function below would be 'disable_kvm' instead of enabling it?

Quoted 16 lines

/arch/x86/kvm/x86.c renamed from linux-3.1.5/arch/x86/kvm/x86.c +9 -1

+
+static int __init enable_kvm_tmem(char *s)
+{
+	kvm_tmem_enabled = 1;
+	return 1;
+}
+
+__setup("kvmtmem", enable_kvm_tmem);
+
 static int __init zcache_init(void)
 {
 	int ret = 0;
-
 #ifdef CONFIG_SYSFS
 	ret = sysfs_create_group(mm_kobj, &zcache_attr_group);
 	if (ret) {

From: Akshay Karle [mailto:[email protected]]
Subject: Re: [RFC 1/2] kvm: host-side changes for tmem on KVM

Quoted 6 lines

Quoted 4 lines

Working at host:
Once the guest exits to the kvm host, the host determines that the guest exited to perform some tmem operation(done at kvm_emulate_hypercall)and then we use zcache to implement this required operations(performed by kvm_pv_tmem_op).

Do you need any modifications to the Kconfig file to reflect the KVM dependency?

Yeah, the Kconfig of zcache now needs to be modified to reflect the KVM and the KVM_INTEL or KVM_AMD dependency. The Kconfig can also be modified to support tmem for the guest kernels, as the zcache code is not at all used in the guest, only the kvm-tmem, cleancache and frontswap(if present) code is used in the guest. But, in the host its necessary to enable zcache. So the Kconfig can be modified to have the following new field:
"Transcendent memory support for kvm guests".... Which would compile just the kvm-tmem code. And we can add Cleancache and Frontswap as dependencies(similar to zcache).

But it should be made sure that the host kernel has zcache enabled..

From: Akshay Karle [mailto:[email protected]]
Subject: Re: [RFC 1/2] kvm: host-side changes for tmem on KVM

Quoted 7 lines

/drivers/staging/zcache/zcache-main.c static struct zv_hdr *zv_create(struct x · line 672

Quoted 5 lines

/drivers/staging/zcache/zcache-main.c renamed from linux-3.1.5/drivers/staging/zcache/zcache-main.c +0 -1

@@ -669,7 +670,6 @@ static struct zv_hdr *zv_create(struct x
      int chunks = (alloc_size + (CHUNK_SIZE - 1)) >> CHUNK_SHIFT;
      int ret;
 
-     BUG_ON(!irqs_disabled());

Can you explain why?

Zcache is by default used in the non-virtualized environment for page compression. Whenever a page is to be evicted from the page cache the spin_lock_irq is held on the page mapping. To ensure that this is done, the BUG_ON(!irqs_disabled()) was used.
But now the situation is different, we are using zcache functions for kvm VM's. So if any page of the guest is to be evicted the irqs should be disabled in just that guest and not the host, so we removed the BUG_ON(!irqs_disabled()); line.

Quoted 11 lines

From: Akshay Karle [mailto:[email protected]]
Subject: Re: [RFC 1/2] kvm: host-side changes for tmem on KVM

Quoted 7 lines

/drivers/staging/zcache/zcache-main.c static struct zv_hdr *zv_create(struct x · line 672

Quoted 5 lines

/drivers/staging/zcache/zcache-main.c renamed from linux-3.1.5/drivers/staging/zcache/zcache-main.c +0 -1

@@ -669,7 +670,6 @@ static struct zv_hdr *zv_create(struct x
      int chunks = (alloc_size + (CHUNK_SIZE - 1)) >> CHUNK_SHIFT;
      int ret;
 
-     BUG_ON(!irqs_disabled());

Can you explain why?

Quoted 6 lines

Zcache is by default used in the non-virtualized environment for page compression. Whenever a page is to be evicted from the page cache the spin_lock_irq is held on the page mapping. To ensure that this is done, the BUG_ON(!irqs_disabled()) was used.
But now the situation is different, we are using zcache functions for kvm VM's. So if any page of the guest is to be evicted the irqs should be disabled in just that guest and not the host, so we removed the BUG_ON(!irqs_disabled()); line.

I think irqs may still need to be disabled (in your code by the caller) since the tmem code (in tmem.c) takes spinlocks with this assumption. I'm not sure since I don't know what can occur with scheduling a kvm guest during an interrupt... can a different vcpu of the same guest be scheduled on this same host pcpu?

Dan

From: Akshay Karle [mailto:[email protected]]
Subject: RE: [RFC 1/2] kvm: host-side changes for tmem on KVM

Quoted 18 lines

Quoted 11 lines

From: Akshay Karle [mailto:[email protected]]
Subject: Re: [RFC 1/2] kvm: host-side changes for tmem on KVM

Quoted 7 lines

/drivers/staging/zcache/zcache-main.c static struct zv_hdr *zv_create(struct x · line 672

Quoted 5 lines

/drivers/staging/zcache/zcache-main.c renamed from linux-3.1.5/drivers/staging/zcache/zcache-main.c +0 -1

@@ -669,7 +670,6 @@ static struct zv_hdr *zv_create(struct x
      int chunks = (alloc_size + (CHUNK_SIZE - 1)) >> CHUNK_SHIFT;
      int ret;
 
-     BUG_ON(!irqs_disabled());

Can you explain why?

Quoted 6 lines

Zcache is by default used in the non-virtualized environment for page compression. Whenever a page is to be evicted from the page cache the spin_lock_irq is held on the page mapping. To ensure that this is done, the BUG_ON(!irqs_disabled()) was used.
But now the situation is different, we are using zcache functions for kvm VM's. So if any page of the guest is to be evicted the irqs should be disabled in just that guest and not the host, so we removed the BUG_ON(!irqs_disabled()); line.

Quoted 5 lines

I think irqs may still need to be disabled (in your code by the caller) since the tmem code (in tmem.c) takes spinlocks with this assumption. I'm not sure since I don't know what can occur with scheduling a kvm guest during an interrupt... can a different vcpu of the same guest be scheduled on this same host pcpu?

The irqs are disabled but only in the guest kernel not in the host. We tried adding the spin_lock_irq code into the host but that was resulting in host panic as the lock is being taken on the entire mapping. If the irqs are disabled in the guest, is there a need to disable them on the host as well? Because the mappings maybe different in the host and the guest.