linux-kernel - [PATCH 5.11 02/31] KVM: x86/mmu: Expand on the comment in kvm_vcpu_ad_need_write

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Date:   Fri, 19 Mar 2021 13:18:56 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.11 02/31] KVM: x86/mmu: Expand on the comment in kvm_vcpu_ad_need_write_protect()

From: Sean Christopherson <seanjc@...gle.com>

[ 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@...gle.com>
Message-Id: <20210213005015.1651772-8-seanjc@...gle.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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;
 }
-- 
2.30.1