linux-kernel - [RFC PATCH 20/24] KVM: nSVM: Do not reset TLB

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Message-ID: <20250326194423.3717668-1-yosry.ahmed@linux.dev>
Date: Wed, 26 Mar 2025 19:44:19 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
	Jim Mattson <jmattson@...gle.com>,
	Maxim Levitsky <mlevitsk@...hat.com>,
	Vitaly Kuznetsov <vkuznets@...hat.com>,
	Rik van Riel <riel@...riel.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	x86@...nel.org,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: [RFC PATCH 20/24] KVM: nSVM: Do not reset TLB_CONTROL in VMCB02 on nested entry

TLB_CONTROL is reset to TLB_CONTROL_DO_NOTHING on nested transitions to
L2 in nested_vmcb02_prepare_control(). This is unnecessary because:
- TLB_CONTROL is set in vcpu_enter_guest() if needed when servicing a
  TLB flush request (by svm_flush_tlb_asid()).
- TLB_CONTROL is reset to TLB_CONTROL_DO_NOTHING after the guest is run
  in svm_cpu_run().

Hence, at the point where nested_vmcb02_prepare_control() is called,
TLB_CONTROL should have already be set to TLB_CONTROL_DO_NOTHING by
svm_cpu_run() after L1 runs.

There is a TODO in nested_svm_transition_tlb_flush() about this reset
crushing pending TLB flushes. Remove it, as the reset is not really
crushing anything as explained above.

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
---
 arch/x86/kvm/svm/nested.c | 10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index ca8db246ac050..544913461693c 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -511,12 +511,7 @@ static void nested_svm_entry_tlb_flush(struct kvm_vcpu *vcpu)
 		svm->nested.last_asid = svm->nested.ctl.asid;
 		kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu);
 	}
-	/*
-	 * TODO: optimize unconditional TLB flush/MMU sync.  A partial list of
-	 * things to fix before this can be conditional:
-	 *
-	 *  - Don't crush a pending TLB flush in vmcb02 on nested VMRUN
-	 */
+	/* TODO: optimize unconditional TLB flush/MMU sync */
 	kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
 	kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
 }
@@ -710,9 +705,6 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
 	vmcb02->control.msrpm_base_pa = vmcb01->control.msrpm_base_pa;
 	vmcb02->control.asid = svm_nested_asid(vcpu->kvm);
 
-	/* Also overwritten later if necessary.  */
-	vmcb_clr_flush_asid(vmcb02);
-
 	/* nested_cr3.  */
 	if (nested_npt_enabled(svm))
 		nested_svm_init_mmu_context(vcpu);
-- 
2.49.0.395.g12beb8f557-goog