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Commit 914ebccd authored by Takuya Yoshikawa's avatar Takuya Yoshikawa Committed by Avi Kivity
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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. In my test, the improved ioctl
was about 4 to 10 times faster than the original one for clean slots.
Furthermore, reducing memory allocations and copies will produce good
effects to caches too.

Signed-off-by: default avatarTakuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: default avatarAvi Kivity <avi@redhat.com>
parent c332c83a

arch/x86/kvm/x86.c

+23 −14
Original line number Diff line number Diff line
@@ -2797,7 +2797,6 @@ 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);
@@ -2812,27 +2811,30 @@ 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;
@@ -2841,13 +2843,20 @@ 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 = 0;

	if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n))

		r = -EFAULT;

out_free:

		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;

out:

	mutex_unlock(&kvm->slots_lock);

	return r;