linux-kernel - [PATCH 09/22] KVM: x86/mmu: Try "unprotect for retry" iff there are indirect SPs

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Message-ID: <20240809190319.1710470-10-seanjc@google.com>
Date: Fri,  9 Aug 2024 12:03:06 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Peter Gonda <pgonda@...gle.com>, Michael Roth <michael.roth@....com>, 
	Vishal Annapurve <vannapurve@...gle.com>, Ackerly Tng <ackerleytng@...gle.com>
Subject: [PATCH 09/22] KVM: x86/mmu: Try "unprotect for retry" iff there are
 indirect SPs

Try to unprotect shadow pages if and only if indirect_shadow_pages is non-
zero, i.e. iff there is at least one protected such shadow page.  Pre-
checking indirect_shadow_pages avoids taking mmu_lock for write when the
gfn is write-protected by a third party, i.e. not for KVM shadow paging,
and in the *extremely* unlikely case that a different task has already
unprotected the last shadow page.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/mmu.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 09a42dc1fe5a..358294889baa 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2736,6 +2736,9 @@ bool kvm_mmu_unprotect_gfn_and_retry(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa)
 	gpa_t gpa = cr2_or_gpa;
 	bool r;
 
+	if (!vcpu->kvm->arch.indirect_shadow_pages)
+		return false;
+
 	if (!vcpu->arch.mmu->root_role.direct)
 		gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2_or_gpa, NULL);
 
-- 
2.46.0.76.ge559c4bf1a-goog