linux-kernel - [RFC PATCH 11/24] KVM: nSVM: Use a separate ASID for nested guests

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Message-ID: <20250326193619.3714986-12-yosry.ahmed@linux.dev>
Date: Wed, 26 Mar 2025 19:36:06 +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 11/24] KVM: nSVM: Use a separate ASID for nested guests

The per-VM ASID is currently shared by both L1 and L2 guests. That ASID
is currently flushed on every transition between L1 and L2.

Allocate and track a separate ASID per-VM for nested guests. This is in
preparation for doing fine-grained TLB flushes on nested transitions
instead of unconditional full flushes.

Nested ASIDs are still not fully maintained (e.g. a remote flush will
only flush the current ASID), so keep the TLB flush on every transition
until this is sorted out in following changes.

Add a helper to get the ASID associated with a specific VMCB and use it
instead of directly reading the VM's ASID. This transparently uses L2's
ASID when an L2 guest is being run.

L1's ASID is flushed on KVM_REQ_TLB_FLUSH_GUEST if it is the active
context, so remove the TODO in nested_svm_transition_tlb_flush() about
it.

Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
---
 arch/x86/kvm/svm/nested.c |  8 ++++++--
 arch/x86/kvm/svm/svm.c    | 13 +++++++++++--
 arch/x86/kvm/svm/svm.h    |  3 ++-
 3 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 81184b2fb27fd..75223869aa8c6 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -495,7 +495,6 @@ static void nested_svm_transition_tlb_flush(struct kvm_vcpu *vcpu)
 	 *  - Honor L1's request to flush an ASID on nested VMRUN
 	 *  - Sync nested NPT MMU on VMRUN that flushes L2's ASID[*]
 	 *  - Don't crush a pending TLB flush in vmcb02 on nested VMRUN
-	 *  - Flush L1's ASID on KVM_REQ_TLB_FLUSH_GUEST
 	 *
 	 * [*] Unlike nested EPT, SVM's ASID management can invalidate nested
 	 *     NPT guest-physical mappings on VMRUN.
@@ -677,7 +676,7 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
 	vmcb02->control.nested_ctl = vmcb01->control.nested_ctl;
 	vmcb02->control.iopm_base_pa = vmcb01->control.iopm_base_pa;
 	vmcb02->control.msrpm_base_pa = vmcb01->control.msrpm_base_pa;
-	vmcb02->control.asid = svm_asid(vcpu->kvm);
+	vmcb02->control.asid = svm_nested_asid(vcpu->kvm);
 
 	/* Also overwritten later if necessary.  */
 	vmcb_clr_flush_asid(vmcb02);
@@ -1179,6 +1178,7 @@ static void nested_svm_triple_fault(struct kvm_vcpu *vcpu)
 
 int svm_allocate_nested(struct vcpu_svm *svm)
 {
+	struct kvm_svm *kvm_svm = to_kvm_svm(svm->vcpu.kvm);
 	struct page *vmcb02_page;
 
 	if (svm->nested.initialized)
@@ -1196,6 +1196,10 @@ int svm_allocate_nested(struct vcpu_svm *svm)
 	svm_vcpu_init_msrpm(&svm->vcpu, svm->nested.msrpm);
 
 	svm->nested.initialized = true;
+
+	if (!kvm_svm->nested_asid)
+		kvm_svm->nested_asid = kvm_svm->asid;
+
 	return 0;
 
 err_free_vmcb02:
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index f028d006f69dc..e664d8428c792 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1225,17 +1225,26 @@ static inline void init_vmcb_after_set_cpuid(struct kvm_vcpu *vcpu)
 	}
 }
 
-unsigned int svm_asid(struct kvm *kvm)
+unsigned int svm_nested_asid(struct kvm *kvm)
+{
+	return to_kvm_svm(kvm)->nested_asid;
+}
+
+static unsigned int svm_asid(struct kvm *kvm)
 {
 	return to_kvm_svm(kvm)->asid;
 }
 
 static unsigned int svm_get_current_asid(struct vcpu_svm *svm)
 {
-	struct kvm *kvm = svm->vcpu.kvm;
+	struct kvm_vcpu *vcpu = &svm->vcpu;
+	struct kvm *kvm = vcpu->kvm;
 
 	if (sev_guest(kvm))
 		return sev_get_asid(kvm);
+	if (is_guest_mode(vcpu))
+		return svm_nested_asid(kvm);
+	WARN_ON_ONCE(svm->current_vmcb != &svm->vmcb01);
 	return svm_asid(kvm);
 }
 
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 436b7e83141b9..e67e3a64e92f7 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -118,6 +118,7 @@ struct kvm_svm {
 	struct kvm kvm;
 
 	unsigned int asid;
+	unsigned int nested_asid;
 
 	/* Struct members for AVIC */
 	u32 avic_vm_id;
@@ -651,7 +652,7 @@ void svm_complete_interrupt_delivery(struct kvm_vcpu *vcpu, int delivery_mode,
 				     int trig_mode, int vec);
 bool svm_register_asid(unsigned int asid);
 void svm_unregister_asid(unsigned int asid);
-unsigned int svm_asid(struct kvm *kvm);
+unsigned int svm_nested_asid(struct kvm *kvm);
 
 /* nested.c */
 
-- 
2.49.0.395.g12beb8f557-goog