linux-kernel - [PATCH 4/7] KVM: SVM: Flush everything if FLUSHBYASID is not available

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Message-ID: <20250313215540.4171762-5-yosry.ahmed@linux.dev>
Date: Thu, 13 Mar 2025 21:55:37 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
	Maxim Levitsky <mlevitsk@...hat.com>,
	x86@...nel.org,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: [PATCH 4/7] KVM: SVM: Flush everything if FLUSHBYASID is not available

Currently, if FLUSHBYASID is not available when performing a TLB flush,
the fallback is decrementing the ASID generation to trigger allocating a
new ASID. In preparation for using a static ASID per VM, just fallback
to flushing everything if FLUSHBYASID is not available. This is probably
worse from a performance perspective, but FLUSHBYASID has been around
for ~15 years and it's not worth carrying the complexity.

The fallback logic is moved within svm_vmcb_set_flush_asid(), as more
callers will be added and will need the fallback as well.

Suggested-by: Sean Christopherson <seanjc@...gle.com>

Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
---
 arch/x86/kvm/svm/svm.c | 5 +----
 arch/x86/kvm/svm/svm.h | 5 ++++-
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 8c90686a33f44..e5064fbefb822 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4005,10 +4005,7 @@ static void svm_flush_tlb_asid(struct kvm_vcpu *vcpu)
 	 * unconditionally does a TLB flush on both nested VM-Enter and nested
 	 * VM-Exit (via kvm_mmu_reset_context()).
 	 */
-	if (static_cpu_has(X86_FEATURE_FLUSHBYASID))
-		svm_vmcb_set_flush_asid(svm->vmcb);
-	else
-		svm->current_vmcb->asid_generation--;
+	svm_vmcb_set_flush_asid(svm->vmcb);
 }
 
 static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 9fd5b249b9c19..0f6426809e1b9 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -645,7 +645,10 @@ void svm_complete_interrupt_delivery(struct kvm_vcpu *vcpu, int delivery_mode,
 
 static inline void svm_vmcb_set_flush_asid(struct vmcb *vmcb)
 {
-	vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ASID;
+	if (static_cpu_has(X86_FEATURE_FLUSHBYASID))
+		vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ASID;
+	else
+		vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ALL_ASID;
 }
 
 static inline void svm_vmcb_clear_flush_asid(struct vmcb *vmcb)
-- 
2.49.0.rc1.451.g8f38331e32-goog