[PATCH v2 2/2] KVM: SVM: Re-load current, not host, TSC_AUX on #VMEXIT from SEV-ES guest

[Sean Christopherson <seanjc@google.com>]

From: Hou Wenlong <houwenlong.hwl@antgroup.com>

Prior to running an SEV-ES guest, set TSC_AUX in the host save area to the
current value in hardware, as tracked by the user return infrastructure,
instead of always loading the host's desired value for the CPU.  If the
pCPU is also running a non-SEV-ES vCPU, loading the host's value on #VMEXIT
could clobber the other vCPU's value, e.g. if the SEV-ES vCPU preempted
the non-SEV-ES vCPU, in which case KVM expects the other vCPU's TSC_AUX
value to be resident in hardware.

Note, unlike TDX, which blindly _zeroes_ TSC_AUX on TD-Exit, SEV-ES CPUs
can load an arbitrary value.  Stuff the current value in the host save
area instead of refreshing the user return cache so that KVM doesn't need
to track whether or not the vCPU actually enterred the guest and thus
loaded TSC_AUX from the host save area.

Fixes: 916e3e5f26ab ("KVM: SVM: Do not use user return MSR support for virtualized TSC_AUX")
Cc: stable@vger.kernel.org
Suggested-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
[sean: handle the SEV-ES case in sev_es_prepare_switch_to_guest()]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/sev.c | 14 +++++++++++++-
 arch/x86/kvm/svm/svm.c | 26 +++++++-------------------
 arch/x86/kvm/svm/svm.h |  4 +++-
 3 files changed, 23 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index cce48fff2e6c..95767b9d0d55 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4664,7 +4664,9 @@ int sev_vcpu_create(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
-void sev_es_prepare_switch_to_guest(struct vcpu_svm *svm, struct sev_es_save_area *hostsa)
+void sev_es_prepare_switch_to_guest(struct vcpu_svm *svm,
+				    struct sev_es_save_area *hostsa,
+				    int tsc_aux_uret_slot)
 {
 	struct kvm *kvm = svm->vcpu.kvm;
 
@@ -4712,6 +4714,16 @@ void sev_es_prepare_switch_to_guest(struct vcpu_svm *svm, struct sev_es_save_are
 		hostsa->dr2_addr_mask = amd_get_dr_addr_mask(2);
 		hostsa->dr3_addr_mask = amd_get_dr_addr_mask(3);
 	}
+
+	/*
+	 * TSC_AUX is always virtualized for SEV-ES guests when the feature is
+	 * available, i.e. TSC_AUX is loaded on #VMEXIT from the host save area.
+	 * Set the save area to the current hardware value, i.e. the current
+	 * user return value, so that the correct value is restored on #VMEXIT.
+	 */
+	if (cpu_feature_enabled(X86_FEATURE_V_TSC_AUX) &&
+	    !WARN_ON_ONCE(tsc_aux_uret_slot < 0))
+		hostsa->tsc_aux = kvm_get_user_return_msr(tsc_aux_uret_slot);
 }
 
 void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 67f4eed01526..662cf680faf7 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -577,18 +577,6 @@ static int svm_enable_virtualization_cpu(void)
 
 	amd_pmu_enable_virt();
 
-	/*
-	 * If TSC_AUX virtualization is supported, TSC_AUX becomes a swap type
-	 * "B" field (see sev_es_prepare_switch_to_guest()) for SEV-ES guests.
-	 * Since Linux does not change the value of TSC_AUX once set, prime the
-	 * TSC_AUX field now to avoid a RDMSR on every vCPU run.
-	 */
-	if (boot_cpu_has(X86_FEATURE_V_TSC_AUX)) {
-		u32 __maybe_unused msr_hi;
-
-		rdmsr(MSR_TSC_AUX, sev_es_host_save_area(sd)->tsc_aux, msr_hi);
-	}
-
 	return 0;
 }
 
@@ -1400,16 +1388,17 @@ static void svm_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
 	 */
 	vmsave(sd->save_area_pa);
 	if (sev_es_guest(vcpu->kvm))
-		sev_es_prepare_switch_to_guest(svm, sev_es_host_save_area(sd));
+		sev_es_prepare_switch_to_guest(svm, sev_es_host_save_area(sd),
+					       tsc_aux_uret_slot);
 
 	if (tsc_scaling)
 		__svm_write_tsc_multiplier(vcpu->arch.tsc_scaling_ratio);
 
 	/*
-	 * TSC_AUX is always virtualized for SEV-ES guests when the feature is
-	 * available. The user return MSR support is not required in this case
-	 * because TSC_AUX is restored on #VMEXIT from the host save area
-	 * (which has been initialized in svm_enable_virtualization_cpu()).
+	 * TSC_AUX is always virtualized (context switched by hardware) for
+	 * SEV-ES guests when the feature is available.  For non-SEV-ES guests,
+	 * context switch TSC_AUX via the user_return MSR infrastructure (not
+	 * all CPUs support TSC_AUX virtualization).
 	 */
 	if (likely(tsc_aux_uret_slot >= 0) &&
 	    (!boot_cpu_has(X86_FEATURE_V_TSC_AUX) || !sev_es_guest(vcpu->kvm)))
@@ -3004,8 +2993,7 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 		 * TSC_AUX is always virtualized for SEV-ES guests when the
 		 * feature is available. The user return MSR support is not
 		 * required in this case because TSC_AUX is restored on #VMEXIT
-		 * from the host save area (which has been initialized in
-		 * svm_enable_virtualization_cpu()).
+		 * from the host save area.
 		 */
 		if (boot_cpu_has(X86_FEATURE_V_TSC_AUX) && sev_es_guest(vcpu->kvm))
 			break;
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 5d39c0b17988..4fda677e8ab3 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -831,7 +831,9 @@ void sev_vcpu_after_set_cpuid(struct vcpu_svm *svm);
 int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in);
 void sev_es_recalc_msr_intercepts(struct kvm_vcpu *vcpu);
 void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector);
-void sev_es_prepare_switch_to_guest(struct vcpu_svm *svm, struct sev_es_save_area *hostsa);
+void sev_es_prepare_switch_to_guest(struct vcpu_svm *svm,
+				    struct sev_es_save_area *hostsa,
+				    int tsc_aux_uret_slot);
 void sev_es_unmap_ghcb(struct vcpu_svm *svm);
 
 #ifdef CONFIG_KVM_AMD_SEV
-- 
2.51.0.470.ga7dc726c21-goog