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

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

[Sean Christopherson]

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@google.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

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

[Sean Christopherson]

On Wed, 27 Nov 2024 15:53:12 -0800, Sean Christopherson wrote:
> 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.
> 
> [...]

Applied to kvm-x86 svm, thanks!

[1/1] KVM: SVM: Remove redundant TLB flush on guest CR4.PGE change
      https://github.com/kvm-x86/linux/commit/036e78a942b4

--
https://github.com/kvm-x86/linux/tree/next