linux-kernel - Re: [PATCH v10 3/9] KVM: x86: Use KVM-governed feature framework to track "LAM enabled"

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Message-ID: <340eab5f346e2581bcb252e975df6724e9f7afa4.camel@intel.com>
Date:   Wed, 16 Aug 2023 23:03:12 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>
CC:     "Gao, Chao" <chao.gao@...el.com>,
        "Zeng, Guang" <guang.zeng@...el.com>,
        "David.Laight@...lab.com" <David.Laight@...lab.com>,
        "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robert.hu@...ux.intel.com" <robert.hu@...ux.intel.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v10 3/9] KVM: x86: Use KVM-governed feature framework to
 track "LAM enabled"

On Wed, 2023-08-16 at 14:33 -0700, Sean Christopherson wrote:
> On Wed, Aug 16, 2023, Kai Huang wrote:
> > 
> > > > > --- a/arch/x86/kvm/vmx/vmx.c
> > > > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > > > @@ -7783,6 +7783,9 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > > > >   		vmx->msr_ia32_feature_control_valid_bits &=
> > > > >   			~FEAT_CTL_SGX_LC_ENABLED;
> > > > >   
> > > > > +	if (boot_cpu_has(X86_FEATURE_LAM))
> > > > > +		kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_LAM);
> > > > > +
> > > > If you want to use boot_cpu_has(), it's better to be done at your last patch to
> > > > only set the cap bit when boot_cpu_has() is true, I suppose.
> > > Yes, but new version of kvm_governed_feature_check_and_set() of 
> > > KVM-governed feature framework will check against kvm_cpu_cap_has() as well.
> > > I will remove the if statement and call 
> > > kvm_governed_feature_check_and_set()  directly.
> > > https://lore.kernel.org/kvm/20230815203653.519297-2-seanjc@google.com/
> > > 
> > 
> > I mean kvm_cpu_cap_has() checks against the host CPUID directly while here you
> > are using boot_cpu_has().  They are not the same.  
> > 
> > If LAM should be only supported when boot_cpu_has() is true then it seems you
> > can just only set the LAM cap bit when boot_cpu_has() is true.  As you also
> > mentioned above the kvm_governed_feature_check_and_set() here internally does
> > kvm_cpu_cap_has().
> 
> That's covered by the last patch:
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index e961e9a05847..06061c11d74d 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -677,7 +677,7 @@ void kvm_set_cpu_caps(void)
>         kvm_cpu_cap_mask(CPUID_7_1_EAX,
>                 F(AVX_VNNI) | F(AVX512_BF16) | F(CMPCCXADD) |
>                 F(FZRM) | F(FSRS) | F(FSRC) |
> -               F(AMX_FP16) | F(AVX_IFMA)
> +               F(AMX_FP16) | F(AVX_IFMA) | F(LAM)
>         );
>  
>         kvm_cpu_cap_init_kvm_defined(CPUID_7_1_EDX,
> 

Ah I missed this piece of code in kvm_set_cpu_caps():

        memcpy(&kvm_cpu_caps, &boot_cpu_data.x86_capability,
               sizeof(kvm_cpu_caps) - (NKVMCAPINTS * sizeof(*kvm_cpu_caps)));

which makes sure KVM only reports one feature when it is set in boot_cpu_data,
which is obviously more reasonable.

Sorry for the noise :-)