[PATCH 2/3] KVM: x86: avoid unnecessary rmap walks when creating/moving slots

From: Anthony Yznaga
Date: Tue Jun 02 2020 - 16:09:16 EST

On large memory guests it has been observed that creating a memslot
for a very large range can take noticeable amount of time.
Investigation showed that the time is spent walking the rmaps to update
existing sptes to remove write access or set/clear dirty bits to support
dirty logging. These rmap walks are unnecessary when creating or moving
a memslot. A newly created memslot will not have any existing mappings,
and the existing mappings of a moved memslot will have been invalidated
and flushed. Any mappings established once the new/moved memslot becomes
visible will be set using the properties of the new slot.

Signed-off-by: Anthony Yznaga <anthony.yznaga@xxxxxxxxxx>
arch/x86/kvm/x86.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 23fd888e52ee..d211c8ced6bb 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10138,7 +10138,7 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
* FIXME: const-ify all uses of struct kvm_memory_slot.
- if (change != KVM_MR_DELETE)
+ if (change == KVM_MR_FLAGS_ONLY)
kvm_mmu_slot_apply_flags(kvm, (struct kvm_memory_slot *) new);

/* Free the arrays associated with the old memslot. */