[PATCH 5.11 02/31] KVM: x86/mmu: Expand on the comment in kvm_vcpu_ad_need_write_protect()

From: Greg Kroah-Hartman
Date: Fri Mar 19 2021 - 08:21:45 EST

From: Sean Christopherson <seanjc@xxxxxxxxxx>

[ Upstream commit 2855f98265dc579bd2becb79ce0156d08e0df813 ]

Expand the comment about need to use write-protection for nested EPT
when PML is enabled to clarify that the tagging is a nop when PML is
_not_ enabled. Without the clarification, omitting the PML check looks
wrong at first^Wfifth glance.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Message-Id: <20210213005015.1651772-8-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
arch/x86/kvm/mmu/mmu_internal.h | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index bfc6389edc28..8404145fb179 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -79,7 +79,10 @@ static inline bool kvm_vcpu_ad_need_write_protect(struct kvm_vcpu *vcpu)
* When using the EPT page-modification log, the GPAs in the log
* would come from L2 rather than L1. Therefore, we need to rely
* on write protection to record dirty pages. This also bypasses
- * PML, since writes now result in a vmexit.
+ * PML, since writes now result in a vmexit. Note, this helper will
+ * tag SPTEs as needing write-protection even if PML is disabled or
+ * unsupported, but that's ok because the tag is consumed if and only
+ * if PML is enabled. Omit the PML check to save a few uops.
return vcpu->arch.mmu == &vcpu->arch.guest_mmu;