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Subject[RFC][PATCH 1/12 applied today] KVM: x86: avoid unnecessary bitmap allocation when memslot is clean
Although we always allocate a new dirty bitmap in x86's get_dirty_log(),
it is only used as a zero-source of copy_to_user() and freed right after
that when memslot is clean. This patch uses clear_user() instead of doing
this unnecessary zero-source allocation.

Performance improvement: as we can expect easily, the time needed to
allocate a bitmap is completely reduced. Furthermore, we can avoid the
tlb flush triggered by vmalloc() and get some good effects. In my test,
the improved ioctl was about 4 to 10 times faster than the original one
for clean slots.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
---
arch/x86/kvm/x86.c | 37 +++++++++++++++++++++++--------------
1 files changed, 23 insertions(+), 14 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6b2ce1d..b95a211 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2744,7 +2744,6 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
struct kvm_memory_slot *memslot;
unsigned long n;
unsigned long is_dirty = 0;
- unsigned long *dirty_bitmap = NULL;

mutex_lock(&kvm->slots_lock);

@@ -2759,27 +2758,30 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,

n = kvm_dirty_bitmap_bytes(memslot);

- r = -ENOMEM;
- dirty_bitmap = vmalloc(n);
- if (!dirty_bitmap)
- goto out;
- memset(dirty_bitmap, 0, n);
-
for (i = 0; !is_dirty && i < n/sizeof(long); i++)
is_dirty = memslot->dirty_bitmap[i];

/* If nothing is dirty, don't bother messing with page tables. */
if (is_dirty) {
struct kvm_memslots *slots, *old_slots;
+ unsigned long *dirty_bitmap;

spin_lock(&kvm->mmu_lock);
kvm_mmu_slot_remove_write_access(kvm, log->slot);
spin_unlock(&kvm->mmu_lock);

- slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
- if (!slots)
- goto out_free;
+ r = -ENOMEM;
+ dirty_bitmap = vmalloc(n);
+ if (!dirty_bitmap)
+ goto out;
+ memset(dirty_bitmap, 0, n);

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

@@ -2788,13 +2790,20 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
synchronize_srcu_expedited(&kvm->srcu);
dirty_bitmap = old_slots->memslots[log->slot].dirty_bitmap;
kfree(old_slots);
+
+ r = -EFAULT;
+ if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n)) {
+ vfree(dirty_bitmap);
+ goto out;
+ }
+ vfree(dirty_bitmap);
+ } else {
+ r = -EFAULT;
+ if (clear_user(log->dirty_bitmap, n))
+ goto out;
}

r = 0;
- if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n))
- r = -EFAULT;
-out_free:
- vfree(dirty_bitmap);
out:
mutex_unlock(&kvm->slots_lock);
return r;
--
1.7.0.4


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