linux-kernel - Re: [PATCH v3 9/9] KVM: x86: Add XSAVE Support for Architectural LBRs

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Date:   Thu, 4 Mar 2021 08:31:33 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, wei.w.wang@...el.com,
        Borislav Petkov <bp@...en8.de>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 9/9] KVM: x86: Add XSAVE Support for Architectural LBRs

On Thu, Mar 04, 2021, Like Xu wrote:
> On 2021/3/4 2:03, Sean Christopherson wrote:
> >          if (vmx_umip_emulated())
> >                  kvm_cpu_cap_set(X86_FEATURE_UMIP);
> > 
> >          /* CPUID 0xD.1 */
> > -       supported_xss = 0;
> >          if (!cpu_has_vmx_xsaves())
> >                  kvm_cpu_cap_clear(X86_FEATURE_XSAVES);
> 
> if (!cpu_has_vmx_xsaves())
> 	supported_xss = 0;

Argh, I forgot XSAVES has a VMCS control.  That's why kvm_arch_hardware_setup()
clears supported_xss if !XSAVES.  I guess just leave that existing code, but
maybe add a comment.

Paolo, any thoughts on how to keep supported_xss aligned with support_xcr0,
without spreading the logic around too much?

> 	kvm_cpu_cap_clear(X86_FEATURE_XSAVES);
> 
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 7b0adebec1ef..5f9eb1f5b840 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -205,6 +205,8 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
> >                                  | XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
> >                                  | XFEATURE_MASK_PKRU)
> > 
> > +#define KVM_SUPPORTED_XSS      XFEATURE_MASK_LBR
> > +
> >   u64 __read_mostly host_efer;
> >   EXPORT_SYMBOL_GPL(host_efer);
> > 
> > @@ -8037,6 +8039,11 @@ int kvm_arch_init(void *opaque)
> >                  supported_xcr0 = host_xcr0 & KVM_SUPPORTED_XCR0;
> >          }
> > 
> > +       if (boot_cpu_has(X86_FEATURE_XSAVES))
> 
> {
> 
> > +               rdmsrl(MSR_IA32_XSS, host_xss);
> > +               supported_xss = host_xss & KVM_SUPPORTED_XSS;
> > +       }
> > +
> >          if (pi_inject_timer == -1)
> >                  pi_inject_timer = housekeeping_enabled(HK_FLAG_TIMER);
> >   #ifdef CONFIG_X86_64
> > @@ -10412,9 +10419,6 @@ int kvm_arch_hardware_setup(void *opaque)
> > 
> >          rdmsrl_safe(MSR_EFER, &host_efer);
> > 
> > -       if (boot_cpu_has(X86_FEATURE_XSAVES))
> > -               rdmsrl(MSR_IA32_XSS, host_xss);
> > -
> >          r = ops->hardware_setup();
> >          if (r != 0)
> >                  return r;
> > @@ -10422,9 +10426,6 @@ int kvm_arch_hardware_setup(void *opaque)
> >          memcpy(&kvm_x86_ops, ops->runtime_ops, sizeof(kvm_x86_ops));
> >          kvm_ops_static_call_update();
> > 
> > -       if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
> > -               supported_xss = 0;
> > -
> >   #define __kvm_cpu_cap_has(UNUSED_, f) kvm_cpu_cap_has(f)
> >          cr4_reserved_bits = __cr4_reserved_bits(__kvm_cpu_cap_has, UNUSED_);
> >   #undef __kvm_cpu_cap_has
> > 
>