[PATCH 2/2] KVM: x86: pre-allocate one more dirty bitmap to avoid vmalloc() in kvm_vm_ioctl_get_dirty_log()

Takuya Yoshikawa
2010-06-03T13:05:54Z

Currently x86's kvm_vm_ioctl_get_dirty_log() needs to allocate a bitmap by
vmalloc() which will be used in the next logging and this has been causing
bad effects to VGA and live-migration: vmalloc() consumes extra systime,
triggers tlb flush, etc.

This patch resolves this issue by pre-allocating one more bitmap and switching
between the two bitmaps during dirty logging.

Performance improvement:
In the usual Desktop environment, kvm_vm_ioctl_get_dirty_log() get about
twice faster than the original one for graphical updates.

During live-migration with low work load, the improvement ratio was
about three when guest memory was 4GB.

Note:
Though this patch introduces some ifdefs, we tried not to mixing these
with other parts to keep the code as clean as possible.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
---
arch/x86/include/asm/kvm_host.h | 2 ++
arch/x86/kvm/x86.c | 16 +++++-----------
include/linux/kvm_host.h | 3 +++
virt/kvm/kvm_main.c | 12 ++++++++++--
4 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 0cd0f29..c516f1c 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -81,6 +81,8 @@
#define KVM_NR_FIXED_MTRR_REGION 88
#define KVM_NR_VAR_MTRR 8

+#define __KVM_DIRTY_LOG_DOUBLE_BUFFERING
+
extern spinlock_t kvm_lock;
extern struct list_head vm_list;

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5e5cd8d..0871ef8 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2804,18 +2804,15 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
kvm_mmu_slot_remove_write_access(kvm, log->slot);
spin_unlock(&kvm->mmu_lock);

- r = -ENOMEM;
- dirty_bitmap = vmalloc(n);
- if (!dirty_bitmap)
- goto out;
+ dirty_bitmap = memslot->dirty_bitmap_head;
+ if (memslot->dirty_bitmap == dirty_bitmap)
+ dirty_bitmap += n / sizeof(long);
memset(dirty_bitmap, 0, n);

r = -ENOMEM;
slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
- if (!slots) {
- vfree(dirty_bitmap);
+ if (!slots)
goto out;
- }
memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
slots->memslots[log->slot].dirty_bitmap = dirty_bitmap;

@@ -2826,11 +2823,8 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
kfree(old_slots);

r = -EFAULT;
- if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n)) {
- vfree(dirty_bitmap);
+ if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n))
goto out;
- }
- vfree(dirty_bitmap);
} else {
r = -EFAULT;
if (clear_user(log->dirty_bitmap, n))
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 2c62319..199a464 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -118,6 +118,9 @@ struct kvm_memory_slot {
unsigned long flags;
unsigned long *rmap;
unsigned long *dirty_bitmap;
+#ifdef __KVM_DIRTY_LOG_DOUBLE_BUFFERING
+ unsigned long *dirty_bitmap_head;
+#endif
struct {
unsigned long rmap_pde;
int write_count;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index e4f5762..6f73b42 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -443,7 +443,10 @@ static void kvm_destroy_dirty_bitmap(struct kvm_memory_slot *memslot)
{
if (!memslot->dirty_bitmap)
return;
-
+#ifdef __KVM_DIRTY_LOG_DOUBLE_BUFFERING
+ memslot->dirty_bitmap = memslot->dirty_bitmap_head;
+ memslot->dirty_bitmap_head = NULL;
+#endif
vfree(memslot->dirty_bitmap);
memslot->dirty_bitmap = NULL;
}
@@ -535,12 +538,17 @@ static int kvm_vm_release(struct inode *inode, struct file *filp)
static int kvm_create_dirty_bitmap(struct kvm_memory_slot *memslot)
{
unsigned long dirty_bytes = kvm_dirty_bitmap_bytes(memslot);
-
+#ifdef __KVM_DIRTY_LOG_DOUBLE_BUFFERING
+ dirty_bytes *= 2;
+#endif
memslot->dirty_bitmap = vmalloc(dirty_bytes);
if (!memslot->dirty_bitmap)
return -ENOMEM;

memset(memslot->dirty_bitmap, 0, dirty_bytes);
+#ifdef __KVM_DIRTY_LOG_DOUBLE_BUFFERING
+ memslot->dirty_bitmap_head = memslot->dirty_bitmap;
+#endif
return 0;
}

--
1.7.0.4