linux-kernel - [PATCH] KVM: SVM: Remove redundant TLB flush on guest CR4.PGE change

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Message-ID: <20241127235312.4048445-1-seanjc@google.com>
Date: Wed, 27 Nov 2024 15:53:12 -0800
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
Subject: [PATCH] KVM: SVM: Remove redundant TLB flush on guest CR4.PGE change

Drop SVM's direct TLB flush when CR4.PGE is toggled and NPT is enabled, as
KVM already guarantees TLBs are flushed appropriately.

For the call from cr_trap(), kvm_post_set_cr4() requests TLB_FLUSH_GUEST
(which is a superset of TLB_FLUSH_CURRENT) when CR4.PGE is toggled,
regardless of whether or not KVM is using TDP.

The calls from nested_vmcb02_prepare_save() and nested_svm_vmexit() are
checking guest (L2) vs. host (L1) CR4, and so a flush is unnecessary as L2
is defined to use a different ASID (from L1's perspective).

Lastly, the call from svm_set_cr0() passes in the current CR4 value, i.e.
can't toggle PGE.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/svm/svm.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index dd15cc635655..f39724bf26be 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -284,8 +284,6 @@ u32 svm_msrpm_offset(u32 msr)
 	return MSR_INVALID;
 }

-static void svm_flush_tlb_current(struct kvm_vcpu *vcpu);
-
 static int get_npt_level(void)
 {
 #ifdef CONFIG_X86_64
@@ -1921,9 +1919,6 @@ void svm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 	unsigned long host_cr4_mce = cr4_read_shadow() & X86_CR4_MCE;
 	unsigned long old_cr4 = vcpu->arch.cr4;

-	if (npt_enabled && ((old_cr4 ^ cr4) & X86_CR4_PGE))
-		svm_flush_tlb_current(vcpu);
-
 	vcpu->arch.cr4 = cr4;
 	if (!npt_enabled) {
 		cr4 |= X86_CR4_PAE;

base-commit: 4d911c7abee56771b0219a9fbf0120d06bdc9c14
-- 
2.47.0.338.g60cca15819-goog