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Date: Wed, 5 Jan 2022 00:22:25 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: "Christopherson,, Sean" <seanjc@...gle.com>,
"Zhong, Yang" <yang.zhong@...el.com>
CC: "x86@...nel.org" <x86@...nel.org>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
"tglx@...utronix.de" <tglx@...utronix.de>,
"mingo@...hat.com" <mingo@...hat.com>,
"bp@...en8.de" <bp@...en8.de>,
"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
"pbonzini@...hat.com" <pbonzini@...hat.com>,
"corbet@....net" <corbet@....net>,
"shuah@...nel.org" <shuah@...nel.org>,
"Nakajima, Jun" <jun.nakajima@...el.com>,
"jing2.liu@...ux.intel.com" <jing2.liu@...ux.intel.com>,
"Liu, Jing2" <jing2.liu@...el.com>,
"Zeng, Guang" <guang.zeng@...el.com>,
"Wang, Wei W" <wei.w.wang@...el.com>
Subject: RE: [PATCH v4 10/21] kvm: x86: Add emulation for IA32_XFD
> From: Sean Christopherson <seanjc@...gle.com>
> Sent: Wednesday, January 5, 2022 3:33 AM
>
> On Wed, Dec 29, 2021, Yang Zhong wrote:
> > From: Jing Liu <jing2.liu@...el.com>
> >
> > Intel's eXtended Feature Disable (XFD) feature allows the software
> > to dynamically adjust fpstate buffer size for XSAVE features which
> > have large state.
> >
> > Because fpstate has been expanded for all possible dynamic xstates
> > at KVM_SET_CPUID2, emulation of the IA32_XFD MSR is straightforward.
> > For write just call fpu_update_guest_xfd() to update the guest fpu
> > container once all the sanity checks are passed. For read then
> > return the cached value in the container.
> >
> > Signed-off-by: Zeng Guang <guang.zeng@...el.com>
> > Signed-off-by: Wei Wang <wei.w.wang@...el.com>
> > Signed-off-by: Jing Liu <jing2.liu@...el.com>
> > Signed-off-by: Yang Zhong <yang.zhong@...el.com>
> > ---
> > arch/x86/kvm/x86.c | 27 +++++++++++++++++++++++++++
> > 1 file changed, 27 insertions(+)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index e50e97ac4408..36677b754ac9 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1359,6 +1359,7 @@ static const u32 msrs_to_save_all[] = {
> > MSR_F15H_PERF_CTL3, MSR_F15H_PERF_CTL4,
> MSR_F15H_PERF_CTL5,
> > MSR_F15H_PERF_CTR0, MSR_F15H_PERF_CTR1,
> MSR_F15H_PERF_CTR2,
> > MSR_F15H_PERF_CTR3, MSR_F15H_PERF_CTR4,
> MSR_F15H_PERF_CTR5,
> > + MSR_IA32_XFD,
> > };
> >
> > static u32 msrs_to_save[ARRAY_SIZE(msrs_to_save_all)];
> > @@ -3669,6 +3670,19 @@ int kvm_set_msr_common(struct kvm_vcpu
> *vcpu, struct msr_data *msr_info)
> > return 1;
> > vcpu->arch.msr_misc_features_enables = data;
> > break;
> > +#ifdef CONFIG_X86_64
> > + case MSR_IA32_XFD:
> > + if (!msr_info->host_initiated &&
> > + !guest_cpuid_has(vcpu, X86_FEATURE_XFD))
> > + return 1;
> > +
> > + if (data & ~(XFEATURE_MASK_USER_DYNAMIC &
> > + vcpu->arch.guest_supported_xcr0))
> > + return 1;
> > +
> > + fpu_update_guest_xfd(&vcpu->arch.guest_fpu, data);
> > + break;
> > +#endif
> > default:
> > if (kvm_pmu_is_valid_msr(vcpu, msr))
> > return kvm_pmu_set_msr(vcpu, msr_info);
> > @@ -3989,6 +4003,15 @@ int kvm_get_msr_common(struct kvm_vcpu
> *vcpu, struct msr_data *msr_info)
> > case MSR_K7_HWCR:
> > msr_info->data = vcpu->arch.msr_hwcr;
> > break;
> > +#ifdef CONFIG_X86_64
> > + case MSR_IA32_XFD:
> > + if (!msr_info->host_initiated &&
> > + !guest_cpuid_has(vcpu, X86_FEATURE_XFD))
> > + return 1;
> > +
> > + msr_info->data = vcpu->arch.guest_fpu.fpstate->xfd;
> > + break;
> > +#endif
> > default:
> > if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
> > return kvm_pmu_get_msr(vcpu, msr_info);
> > @@ -6422,6 +6445,10 @@ static void kvm_init_msr_list(void)
> > min(INTEL_PMC_MAX_GENERIC,
> x86_pmu.num_counters_gp))
> > continue;
> > break;
> > + case MSR_IA32_XFD:
> > + if (!kvm_cpu_cap_has(X86_FEATURE_XFD))
> > + continue;
>
> I suspect the 32-bit host support is wrong. The kernel's handle_xfd_event()
> checks for 64-bit support in addition to the CPU feature itself, which implies
> that the feature can be reported in boot_cpu_data for 32-bit kernels.
>
> static bool handle_xfd_event(struct pt_regs *regs)
> {
> u64 xfd_err;
> int err;
>
> if (!IS_ENABLED(CONFIG_X86_64)
> || !cpu_feature_enabled(X86_FEATURE_XFD))
> return false;
>
> ...
> }
>
> In this specific case, that means KVM will tell userspace it needs to mgirate
> MSR_IA32_XFD, and then reject attempts to read/write the MSR.
>
> If 32-bit host kernels do not explicitly suppress X86_FEATURE_XFD, then KVM
I didn't find explicit suppress
> needs
> to do:
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 556555537a18..156ce332d55b 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -455,9 +455,11 @@ void kvm_set_cpu_caps(void)
> #ifdef CONFIG_X86_64
> unsigned int f_gbpages = F(GBPAGES);
> unsigned int f_lm = F(LM);
> + unsigned int f_xfd = F(XFD);
> #else
> unsigned int f_gbpages = 0;
> unsigned int f_lm = 0;
> + unsigned int f_xfd = 0;
> #endif
> memset(kvm_cpu_caps, 0, sizeof(kvm_cpu_caps));
>
> @@ -545,7 +547,7 @@ void kvm_set_cpu_caps(void)
> );
>
> kvm_cpu_cap_mask(CPUID_D_1_EAX,
> - F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | F(XSAVES) | F(XFD)
> + F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | F(XSAVES) | f_xfd
> );
>
> kvm_cpu_cap_init_scattered(CPUID_12_EAX,
>
so this change makes sense. will incorporate it in next version.
Thanks
Kevin
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