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References: <20220105123532.12586-1-yang.zhong@intel.com> <20220105123532.12586-13-yang.zhong@intel.com>
 <YdYaH7buoApEVPOg@google.com>
In-Reply-To: <YdYaH7buoApEVPOg@google.com>
From:   Jim Mattson <jmattson@google.com>
Date:   Wed, 5 Jan 2022 14:30:45 -0800
Message-ID: <CALMp9eQi1xU_YgYBTa=zvAkD8=4WAntSFt6V2zcR6G9eQFg2GQ@mail.gmail.com>
Subject: Re: [PATCH v5 12/21] kvm: x86: Intercept #NM for saving IA32_XFD_ERR
To:     Sean Christopherson <seanjc@google.com>
Cc:     Yang Zhong <yang.zhong@intel.com>, x86@kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, pbonzini@redhat.com, corbet@lwn.net,
        shuah@kernel.org, jun.nakajima@intel.com, kevin.tian@intel.com,
        jing2.liu@linux.intel.com, jing2.liu@intel.com,
        guang.zeng@intel.com, wei.w.wang@intel.com
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On Wed, Jan 5, 2022 at 2:22 PM Sean Christopherson <seanjc@google.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?