[PATCH v3 3/4] KVM: x86: Tie Intel and AMD behavior for MSR_TSC_AUX to guest CPU model

Sean Christopherson
2021-04-23T22:34:37Z

Squish the Intel and AMD emulation of MSR_TSC_AUX together and tie it to
the guest CPU model instead of the host CPU behavior. While not strictly
necessary to avoid guest breakage, emulating cross-vendor "architecture"
will provide consistent behavior for the guest, e.g. WRMSR fault behavior
won't change if the vCPU is migrated to a host with divergent behavior.

Note, this also adds a kvm_cpu_has() check on RDTSCP for VMX. For all
practical purposes, the extra check is a nop as VMX's use of user return
MSRs indirectly performs the same check by checking the result of WRMSR.
Technically RDTSCP support can exist in bare metal but not in the VMCS,
but no known Intel CPUs behave this way. In practice, the only scenario
where adding the kvm_cpu_has() check isn't a nop is nested virtualization
scenario where L0 hides the VMCS control from L1 (KVM in this case), and
the L1 userspace VMM has decided to advertise RDTSCP _against_ the
recommendations of KVM_GET_SUPPORTED_CPUID.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/svm.c | 22 +---------------------
arch/x86/kvm/vmx/vmx.c | 13 -------------
arch/x86/kvm/x86.c | 34 ++++++++++++++++++++++++++++++++++
3 files changed, 35 insertions(+), 34 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 71d704f8d569..4c7604fca009 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2672,11 +2672,6 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
msr_info->data |= (u64)svm->sysenter_esp_hi << 32;
break;
case MSR_TSC_AUX:
- if (!boot_cpu_has(X86_FEATURE_RDTSCP))
- return 1;
- if (!msr_info->host_initiated &&
- !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
- return 1;
msr_info->data = svm->tsc_aux;
break;
/*
@@ -2892,13 +2887,6 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
svm->sysenter_esp_hi = guest_cpuid_is_intel(vcpu) ? (data >> 32) : 0;
break;
case MSR_TSC_AUX:
- if (!boot_cpu_has(X86_FEATURE_RDTSCP))
- return 1;
-
- if (!msr->host_initiated &&
- !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
- return 1;
-
/*
* This is rare, so we update the MSR here instead of using
* direct_access_msrs. Doing that would require a rdmsr in
@@ -2906,15 +2894,7 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
*/
wrmsrl(MSR_TSC_AUX, svm->tsc_aux);

- /*
- * Per Intel's SDM, bits 63:32 are reserved, but AMD's APM has
- * incomplete and conflicting architectural behavior. Current
- * AMD CPUs completely ignore bits 63:32, i.e. they aren't
- * reserved and always read as zeros. Emulate AMD CPU behavior
- * to avoid explosions if the vCPU is migrated from an AMD host
- * to an Intel host.
- */
- svm->tsc_aux = (u32)data;
+ svm->tsc_aux = data;
break;
case MSR_IA32_DEBUGCTLMSR:
if (!boot_cpu_has(X86_FEATURE_LBRV)) {
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 6501d66167b8..d3fce53d89ac 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1992,11 +1992,6 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
else
msr_info->data = vmx->pt_desc.guest.addr_a[index / 2];
break;
- case MSR_TSC_AUX:
- if (!msr_info->host_initiated &&
- !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
- return 1;
- goto find_uret_msr;
case MSR_IA32_DEBUGCTLMSR:
msr_info->data = vmcs_read64(GUEST_IA32_DEBUGCTL);
break;
@@ -2312,14 +2307,6 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
else
vmx->pt_desc.guest.addr_a[index / 2] = data;
break;
- case MSR_TSC_AUX:
- if (!msr_info->host_initiated &&
- !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
- return 1;
- /* Check reserved bit, higher 32 bits should be zero */
- if ((data >> 32) != 0)
- return 1;
- goto find_uret_msr;
case MSR_IA32_PERF_CAPABILITIES:
if (data && !vcpu_to_pmu(vcpu)->version)
return 1;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f0d0b6e043ae..95da9b1cabdb 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1610,6 +1610,29 @@ static int __kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data,
* invokes 64-bit SYSENTER.
*/
data = get_canonical(data, vcpu_virt_addr_bits(vcpu));
+ break;
+ case MSR_TSC_AUX:
+ if (!kvm_cpu_cap_has(X86_FEATURE_RDTSCP))
+ return 1;
+
+ if (!host_initiated &&
+ !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
+ return 1;
+
+ /*
+ * Per Intel's SDM, bits 63:32 are reserved, but AMD's APM has
+ * incomplete and conflicting architectural behavior. Current
+ * AMD CPUs completely ignore bits 63:32, i.e. they aren't
+ * reserved and always read as zeros. Enforce Intel's reserved
+ * bits check if and only if the guest CPU is Intel, and clear
+ * the bits in all other cases. This ensures cross-vendor
+ * migration will provide consistent behavior for the guest.
+ */
+ if (guest_cpuid_is_intel(vcpu) && (data >> 32) != 0)
+ return 1;
+
+ data = (u32)data;
+ break;
}

msr.data = data;
@@ -1646,6 +1669,17 @@ int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data,
if (!host_initiated && !kvm_msr_allowed(vcpu, index, KVM_MSR_FILTER_READ))
return KVM_MSR_RET_FILTERED;

+ switch (index) {
+ case MSR_TSC_AUX:
+ if (!kvm_cpu_cap_has(X86_FEATURE_RDTSCP))
+ return 1;
+
+ if (!host_initiated &&
+ !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
+ return 1;
+ break;
+ }
+
msr.index = index;
msr.host_initiated = host_initiated;

--
2.31.1.498.g6c1eba8ee3d-goog