linux-kernel - [PATCH] KVM: nSVM: fix nested tsc scaling when not enabled but MSR_AMD64_TSC

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Date:   Mon, 21 Feb 2022 12:33:31 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        Maxim Levitsky <mlevitsk@...hat.com>, stable@...r.kernel.org,
        Dāvis Mosāns <davispuh@...il.com>
Subject: [PATCH] KVM: nSVM: fix nested tsc scaling when not enabled but MSR_AMD64_TSC_RATIO set

For compatibility with userspace the MSR_AMD64_TSC_RATIO can be set
even when the feature is not exposed to the guest, which breaks assumptions
that it has the default value in this case.

Fixes: 5228eb96a487 ("KVM: x86: nSVM: implement nested TSC scaling")
Cc: stable@...r.kernel.org

Reported-by: Dāvis Mosāns <davispuh@...il.com>
Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
---
 arch/x86/kvm/svm/nested.c | 10 ++++------
 arch/x86/kvm/svm/svm.c    |  5 +++--
 arch/x86/kvm/svm/svm.h    |  1 +
 3 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 1218b5a342fc..a2e2436057dc 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -574,14 +574,12 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm)
 	vcpu->arch.tsc_offset = kvm_calc_nested_tsc_offset(
 			vcpu->arch.l1_tsc_offset,
 			svm->nested.ctl.tsc_offset,
-			svm->tsc_ratio_msr);
+			svm_get_l2_tsc_multiplier(vcpu));
 
 	svm->vmcb->control.tsc_offset = vcpu->arch.tsc_offset;
 
-	if (svm->tsc_ratio_msr != kvm_default_tsc_scaling_ratio) {
-		WARN_ON(!svm->tsc_scaling_enabled);
+	if (svm_get_l2_tsc_multiplier(vcpu) != kvm_default_tsc_scaling_ratio)
 		nested_svm_update_tsc_ratio_msr(vcpu);
-	}
 
 	svm->vmcb->control.int_ctl             =
 		(svm->nested.ctl.int_ctl & int_ctl_vmcb12_bits) |
@@ -867,8 +865,7 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
 		vmcb_mark_dirty(svm->vmcb, VMCB_INTERCEPTS);
 	}
 
-	if (svm->tsc_ratio_msr != kvm_default_tsc_scaling_ratio) {
-		WARN_ON(!svm->tsc_scaling_enabled);
+	if (svm_get_l2_tsc_multiplier(vcpu) != kvm_default_tsc_scaling_ratio) {
 		vcpu->arch.tsc_scaling_ratio = vcpu->arch.l1_tsc_scaling_ratio;
 		svm_write_tsc_multiplier(vcpu, vcpu->arch.tsc_scaling_ratio);
 	}
@@ -1264,6 +1261,7 @@ void nested_svm_update_tsc_ratio_msr(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 
+	WARN_ON_ONCE(!svm->tsc_scaling_enabled);
 	vcpu->arch.tsc_scaling_ratio =
 		kvm_calc_nested_tsc_multiplier(vcpu->arch.l1_tsc_scaling_ratio,
 					       svm->tsc_ratio_msr);
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 975be872cd1a..5cf6bb5bfd3e 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -911,11 +911,12 @@ static u64 svm_get_l2_tsc_offset(struct kvm_vcpu *vcpu)
 	return svm->nested.ctl.tsc_offset;
 }
 
-static u64 svm_get_l2_tsc_multiplier(struct kvm_vcpu *vcpu)
+u64 svm_get_l2_tsc_multiplier(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 
-	return svm->tsc_ratio_msr;
+	return svm->tsc_scaling_enabled ? svm->tsc_ratio_msr :
+	       kvm_default_tsc_scaling_ratio;
 }
 
 static void svm_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 852b12aee03d..006571dd30df 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -542,6 +542,7 @@ int nested_svm_check_exception(struct vcpu_svm *svm, unsigned nr,
 			       bool has_error_code, u32 error_code);
 int nested_svm_exit_special(struct vcpu_svm *svm);
 void nested_svm_update_tsc_ratio_msr(struct kvm_vcpu *vcpu);
+u64 svm_get_l2_tsc_multiplier(struct kvm_vcpu *vcpu);
 void svm_write_tsc_multiplier(struct kvm_vcpu *vcpu, u64 multiplier);
 void nested_copy_vmcb_control_to_cache(struct vcpu_svm *svm,
 				       struct vmcb_control_area *control);
-- 
2.26.3