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Date: Tue, 19 Mar 2019 12:37:23 +0800
From: Xiaoyao Li <xiaoyao.li@...ux.intel.com>
To: Sean Christopherson <sean.j.christopherson@...el.com>
Cc: kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
Radim Krčmář <rkrcmar@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
chao.gao@...el.com
Subject: Re: [PATCH v2 2/2] kvm/vmx: Using hardware cpuid faulting to avoid
emulation overhead
On Mon, 2019-03-18 at 09:38 -0700, Sean Christopherson wrote:
> On Mon, Mar 18, 2019 at 07:43:24PM +0800, Xiaoyao Li wrote:
> > Current cpuid faulting of guest is purely emulated in kvm, which exploits
> > CPUID vm exit to inject #GP to guest. However, if host hardware cpu has
> > X86_FEATURE_CPUID_FAULT, we can just use the hardware cpuid faulting for
> > guest to avoid the vm exit overhead.
>
> Heh, I obviously didn't look at this patch before responding to patch 1/2.
>
> > Note: cpuid faulting takes higher priority over CPUID instruction vm
> > exit (Intel SDM vol3.25.1.1).
> >
> > Since cpuid faulting only exists on some Intel's cpu, just apply this
> > optimization to vmx.
> >
> > Signed-off-by: Xiaoyao Li <xiaoyao.li@...ux.intel.com>
> > ---
> > arch/x86/include/asm/kvm_host.h | 2 ++
> > arch/x86/kvm/vmx/vmx.c | 19 +++++++++++++++----
> > arch/x86/kvm/x86.c | 15 ++++++++++++---
> > 3 files changed, 29 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h
> > b/arch/x86/include/asm/kvm_host.h
> > index ce79d7bfe1fd..14cad587b804 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1339,6 +1339,8 @@ 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 2c59e0209e36..6b413e471dca 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -1037,7 +1037,7 @@ static void pt_guest_exit(struct vcpu_vmx *vmx)
> >
> > static void vmx_save_host_cpuid_fault(struct vcpu_vmx *vmx)
> > {
> > - u64 host_val;
> > + u64 host_val, guest_val;
> >
> > if (!boot_cpu_has(X86_FEATURE_CPUID_FAULT))
> > return;
> > @@ -1045,10 +1045,12 @@ static void vmx_save_host_cpuid_fault(struct
> > vcpu_vmx *vmx)
> > rdmsrl(MSR_MISC_FEATURES_ENABLES, host_val);
> > vmx->host_msr_misc_features_enables = host_val;
> >
> > - /* clear cpuid fault bit to avoid it leak to guest */
> > - if (host_val & MSR_MISC_FEATURES_ENABLES_CPUID_FAULT) {
> > + guest_val = vmx->vcpu.arch.msr_misc_features_enables;
> > +
> > + /* we can use the hardware cpuid faulting to avoid emulation overhead */
> > + if ((host_val ^ guest_val) & MSR_MISC_FEATURES_ENABLES_CPUID_FAULT) {
> > wrmsrl(MSR_MISC_FEATURES_ENABLES,
> > - host_val & ~MSR_MISC_FEATURES_ENABLES_CPUID_FAULT);
> > + host_val ^ MSR_MISC_FEATURES_ENABLES_CPUID_FAULT);
> > }
> > }
> >
> > @@ -2057,6 +2059,15 @@ 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 (boot_cpu_has(X86_FEATURE_CPUID_FAULT)) {
> > + if (vmx->loaded_cpu_state)
>
> No need for two separate if statements. And assuming we're checking the
> existing shadow value when loading guest/host state, the WRMSR should
> only be done if the host's value is non-zero.
I'll combine these two if statements into one.
I cannot understand why the WRMSR should only be done if the host's value is
non-zero. I think there is no depedency with host's value, if using the hardware
cpuid faulting. We just need to set the value to real hardware MSR.
> > + 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 434ec113cc79..33a8c95b2f2e 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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