linux-kernel - [PATCH] KVM: x86/mmu: Check for host MMIO exclusion from mem encrypt iff necessary

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Date:   Fri, 15 Apr 2022 00:49:09 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] KVM: x86/mmu: Check for host MMIO exclusion from mem encrypt
 iff necessary

When determining whether or not a SPTE needs to have SME/SEV's memory
encryption flag set, do the moderately expension host MMIO pfn check if
and only if the memory encryption mask is non-zero.

Note, KVM could further optimize the host MMIO checks by making a single
call to kvm_is_mmio_pfn(), but the tdp_enabled path (for EPT's memtype
handling) will likely be split out to a separate flow[*].  At that point,
a better approach would be to shove the call to kvm_is_mmio_pfn() into
VMX code so that AMD+NPT without SME doesn't get hit with an unnecessary
lookup.

[*] https://lkml.kernel.org/r/20220321224358.1305530-3-bgardon@google.com

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/spte.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 4739b53c9734..aab78574e03d 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -139,7 +139,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 	else
 		pte_access &= ~ACC_WRITE_MASK;

-	if (!kvm_is_mmio_pfn(pfn))
+	if (shadow_me_mask && !kvm_is_mmio_pfn(pfn))
 		spte |= shadow_me_mask;

 	spte |= (u64)pfn << PAGE_SHIFT;

base-commit: 150866cd0ec871c765181d145aa0912628289c8a
-- 
2.36.0.rc0.470.gd361397f0d-goog