KVM: pre-allocate one more dirty bitmap to avoid vmalloc()

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 effect 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 two bitmaps during dirty logging.

Performance improvement:
  I measured performance for the case of VGA update by trace-cmd.
  The result was 1.5 times faster than the original one.

  In the case of live migration, the improvement ratio depends on the workload
  and the guest memory size. In general, the larger the memory size is the more
  benefits we get.

Note:
  This does not change other architectures's logic but the allocation size
  becomes twice. This will increase the actual memory consumption only when
  the new size changes the number of pages allocated by vmalloc().

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 5 insertions, 11 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a2a785472431..35f82f2c66f6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3208,18 +3208,15 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
                 struct kvm_memslots *slots, *old_slots;
                 unsigned long *dirty_bitmap;
 
-                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;
                 slots->generation++;
@@ -3235,11 +3232,8 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
                 spin_unlock(&kvm->mmu_lock);
 
                 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))