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

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Date:   Tue,  2 Jun 2020 13:07:29 -0700
From:   Anthony Yznaga <anthony.yznaga@...cle.com>
To:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     pbonzini@...hat.com, sean.j.christopherson@...el.com,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        steven.sistare@...cle.com, anthony.yznaga@...cle.com
Subject: [PATCH 2/3] KVM: x86: avoid unnecessary rmap walks when creating/moving slots

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@...cle.com>
---
 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. */
-- 
2.13.3