linux-kernel - Re: [PATCH v5 12/21] kvm: x86: Intercept #NM for saving IA32_XFD

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Date:   Wed, 5 Jan 2022 14:30:45 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Yang Zhong <yang.zhong@...el.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, pbonzini@...hat.com, corbet@....net,
        shuah@...nel.org, jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, jing2.liu@...el.com,
        guang.zeng@...el.com, wei.w.wang@...el.com
Subject: Re: [PATCH v5 12/21] kvm: x86: Intercept #NM for saving IA32_XFD_ERR

On Wed, Jan 5, 2022 at 2:22 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, Jan 05, 2022, Yang Zhong wrote:
> > @@ -6399,6 +6424,26 @@ static void handle_interrupt_nmi_irqoff(struct kvm_vcpu *vcpu,
> >       kvm_after_interrupt(vcpu);
> >  }
> >
> > +static void handle_nm_fault_irqoff(struct kvm_vcpu *vcpu)
> > +{
> > +     /*
> > +      * Save xfd_err to guest_fpu before interrupt is enabled, so the
> > +      * MSR value is not clobbered by the host activity before the guest
> > +      * has chance to consume it.
> > +      *
> > +      * We should not blindly read xfd_err here, since this exception
>
> Nit, avoid "we", and explain what KVM does (or doesn't) do, not what KVM "should"
> do, e.g. just say
>
>          * Do not blindly read ...
>
> > +      * might be caused by L1 interception on a platform which doesn't
> > +      * support xfd at all.
> > +      *
> > +      * Do it conditionally upon guest_fpu::xfd. xfd_err matters
> > +      * only when xfd contains a non-zero value.
> > +      *
> > +      * Queuing exception is done in vmx_handle_exit. See comment there.
>
> Another nit, it's worth explaining why XFD_ERR needs to be read here regardless
> of is_guest_mode().  E.g.
>
>          * Injecting the #NM back into the guest is handled in the standard path
>          * as an #NM in L2 may be reflected into L1 as a VM-Exit.  Read XFD_ERR
>          * even if the #NM is from L2, as L1 may have exposed XFD to L2.

Do we have tests of L1 passing through XFD to L2?