On 9/15/23 09:51, Sean Christopherson wrote:
On Fri, Sep 15, 2023, Sean Christopherson wrote:
On Thu, Sep 14, 2023, Tom Lendacky wrote:
When the TSC_AUX MSR is virtualized, the TSC_AUX value is swap type "B"
within the VMSA. This means that the guest value is loaded on VMRUN and
the host value is restored from the host save area on #VMEXIT.
Since the value is restored on #VMEXIT, the KVM user return MSR support
for TSC_AUX can be replaced by populating the host save area with current
host value of TSC_AUX. This replaces two WRMSR instructions with a single
RDMSR instruction.
Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
---
arch/x86/kvm/svm/sev.c | 14 +++++++++++++-
arch/x86/kvm/svm/svm.c | 26 ++++++++++++++++----------
arch/x86/kvm/svm/svm.h | 4 +++-
3 files changed, 32 insertions(+), 12 deletions(-)
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 565c9de87c6d..1bbaae2fed96 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2969,6 +2969,7 @@ static void sev_es_init_vmcb_after_set_cpuid(struct vcpu_svm *svm)
if (boot_cpu_has(X86_FEATURE_V_TSC_AUX) &&
(guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP) ||
guest_cpuid_has(vcpu, X86_FEATURE_RDPID))) {
+ svm->v_tsc_aux = true;
set_msr_interception(vcpu, svm->msrpm, MSR_TSC_AUX, 1, 1);
if (guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
svm_clr_intercept(svm, INTERCEPT_RDTSCP);
@@ -3071,8 +3072,10 @@ void sev_es_vcpu_reset(struct vcpu_svm *svm)
sev_enc_bit));
}
-void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa)
+void sev_es_prepare_switch_to_guest(struct vcpu_svm *svm, struct sev_es_save_area *hostsa)
{
+ u32 msr_hi;
+
/*
* All host state for SEV-ES guests is categorized into three swap types
* based on how it is handled by hardware during a world switch:
@@ -3109,6 +3112,15 @@ void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa)
hostsa->dr2_addr_mask = amd_get_dr_addr_mask(2);
hostsa->dr3_addr_mask = amd_get_dr_addr_mask(3);
}
+
+ /*
+ * If TSC_AUX virtualization is enabled, MSR_TSC_AUX is loaded but NOT
+ * saved by the CPU (Type-B). If TSC_AUX is not virtualized, the user
+ * return MSR support takes care of restoring MSR_TSC_AUX. This
+ * exchanges two WRMSRs for one RDMSR.
+ */
+ if (svm->v_tsc_aux)
+ rdmsr(MSR_TSC_AUX, hostsa->tsc_aux, msr_hi);
IIUC, when V_TSC_AUX is supported, SEV-ES guests context switch MSR_TSC_AUX
regardless of what has been exposed to the guest. So rather than condition the
hostsa->tsc_aux update on guest CPUID, just do it if V_TSC_AUX is supported.
And then to avoid the RDMSR, which is presumably the motivation for checking
guest CPUID, grab the host value from user return framework. The host values
are per-CPU, but constant after boot, so the only requirement is that KVM sets
up MSR_TSC_AUX in the user return framework.
Actually, duh. The save area is also per-CPU, so just fill hostsa->tsc_aux in
svm_hardware_setup() and then sev_es_prepare_switch_to_guest() never has to do
anything.
Ah, right, because Linux never changes TSC_AUX post boot. Much simpler.
I'll rework based on the comments and send a v2 series.
Thanks,
Tom
