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Subject[PATCH v3 2/2] x86/vmx: optimize MSR_MISC_FEATURES_ENABLES switch
Date
KVM needs to switch MSR_MISC_FEATURES_ENABLES between host and guest in
every pcpu/vcpu context switch. Since WRMSR is expensive, this patch tries
to save cycles by avoiding WRMSR MSR_MISC_FEATURES_ENABLES whenever possible.

If host's value is zero, nothing needs to be done, since guest can use
kvm emulated cpuid faulting.

If host's value is non-zero, it need not clear MSR_MISC_FEATURES_ENABLES
unconditionally. We can use hardware cpuid faulting if guest's
value is equal to host'value, thus avoid WRMSR
MSR_MISC_FEATURES_ENABLES.

Since hardware cpuid faulting takes higher priority than CPUID vm exit,
it should be updated to hardware while guest wrmsr and hardware cpuid
faulting is used for guest.

Signed-off-by: Xiaoyao Li <xiaoyao.li@linux.intel.com>
---
arch/x86/include/asm/kvm_host.h | 2 ++
arch/x86/kvm/vmx/vmx.c | 13 ++++++++++---
arch/x86/kvm/x86.c | 15 ++++++++++++---
3 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 2c53df4a5a2a..9bcb444b903d 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1343,6 +1343,8 @@ void kvm_lmsw(struct kvm_vcpu *vcpu, unsigned long msw);
void kvm_get_cs_db_l_bits(struct kvm_vcpu *vcpu, int *db, int *l);
int kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr);

+int kvm_supported_msr_misc_features_enables(struct kvm_vcpu *vcpu, u64 data);
+
int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr);
int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr);

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 65aa947947ba..73bb11f74b36 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1035,10 +1035,10 @@ static void vmx_prepare_guest_misc_features_enables(struct vcpu_vmx *vmx)
{
u64 msrval = this_cpu_read(msr_misc_features_shadow);

- if (!msrval)
+ if (!msrval || msrval == vmx->vcpu.arch.msr_misc_features_enables)
return;

- wrmsrl(MSR_MISC_FEATURES_ENABLES, 0ULL);
+ wrmsrl(MSR_MISC_FEATURES_ENABLES, vmx->vcpu.arch.msr_misc_features_enables);
}

void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
@@ -1136,7 +1136,7 @@ static void vmx_load_host_misc_features_enables(struct vcpu_vmx *vmx)
{
u64 msrval = this_cpu_read(msr_misc_features_shadow);

- if (!msrval)
+ if (!msrval || msrval == vmx->vcpu.arch.msr_misc_features_enables)
return;

wrmsrl(MSR_MISC_FEATURES_ENABLES, msrval);
@@ -2043,6 +2043,13 @@ 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_MISC_FEATURES_ENABLES:
+ if (!kvm_supported_msr_misc_features_enables(vcpu, data))
+ return 1;
+ if (this_cpu_read(msr_misc_features_shadow) && vmx->loaded_cpu_state)
+ wrmsrl(MSR_MISC_FEATURES_ENABLES, data);
+ vcpu->arch.msr_misc_features_enables = data;
+ break;
case MSR_TSC_AUX:
if (!msr_info->host_initiated &&
!guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ad1df965574e..d2af90422a51 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2449,6 +2449,17 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
&vcpu->arch.st.steal, sizeof(struct kvm_steal_time));
}

+int kvm_supported_msr_misc_features_enables(struct kvm_vcpu *vcpu, u64 data)
+{
+ if (data & ~MSR_MISC_FEATURES_ENABLES_CPUID_FAULT ||
+ (data & MSR_MISC_FEATURES_ENABLES_CPUID_FAULT &&
+ !supports_cpuid_fault(vcpu)))
+ return 0;
+ else
+ return 1;
+}
+EXPORT_SYMBOL_GPL(kvm_supported_msr_misc_features_enables);
+
int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
{
bool pr = false;
@@ -2669,9 +2680,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
vcpu->arch.msr_platform_info = data;
break;
case MSR_MISC_FEATURES_ENABLES:
- if (data & ~MSR_MISC_FEATURES_ENABLES_CPUID_FAULT ||
- (data & MSR_MISC_FEATURES_ENABLES_CPUID_FAULT &&
- !supports_cpuid_fault(vcpu)))
+ if (!kvm_supported_msr_misc_features_enables(vcpu, data))
return 1;
vcpu->arch.msr_misc_features_enables = data;
break;
--
2.19.1
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