linux-kernel - Re: [PATCH v7 6/7] KVM: x86: Load Guest fpu state when accessing MSRs managed by XSAVES

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Message-ID: <20191009064648.GD27851@local-michael-cet-test>
Date:   Wed, 9 Oct 2019 14:46:48 +0800
From:   Yang Weijiang <weijiang.yang@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v7 6/7] KVM: x86: Load Guest fpu state when accessing
 MSRs managed by XSAVES

On Wed, Oct 02, 2019 at 12:56:30PM -0700, Jim Mattson wrote:
> On Thu, Sep 26, 2019 at 7:17 PM Yang Weijiang <weijiang.yang@...el.com> wrote:
> >
> > From: Sean Christopherson <sean.j.christopherson@...el.com>
> >
 >  /*
> >   * Read or write a bunch of msrs. All parameters are kernel addresses.
> >   *
> > @@ -3009,11 +3017,23 @@ static int __msr_io(struct kvm_vcpu *vcpu, struct kvm_msrs *msrs,
> >                     int (*do_msr)(struct kvm_vcpu *vcpu,
> >                                   unsigned index, u64 *data))
> >  {
> > +       bool fpu_loaded = false;
> >         int i;
> > +       const u64 cet_bits = XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL;
> > +       bool cet_xss = kvm_x86_ops->xsaves_supported() &&
> > +                      (kvm_supported_xss() & cet_bits);
> 
> It seems like I've seen a lot of checks like this. Can this be
> simplified (throughout this series) by sinking the
> kvm_x86_ops->xsaves_supported() check into kvm_supported_xss()? That
> is, shouldn't kvm_supported_xss() return 0 if
> kvm_x86_ops->xsaves_supported() is false?
>
OK, let me add this check, thank you!

> > -       for (i = 0; i < msrs->nmsrs; ++i)
> > +       for (i = 0; i < msrs->nmsrs; ++i) {
> > +               if (!fpu_loaded && cet_xss &&
> > +                   is_xsaves_msr(entries[i].index)) {
> > +                       kvm_load_guest_fpu(vcpu);
> > +                       fpu_loaded = true;
> > +               }
> >                 if (do_msr(vcpu, entries[i].index, &entries[i].data))
> >                         break;
> > +       }
> > +       if (fpu_loaded)
> > +               kvm_put_guest_fpu(vcpu);
> >
> >         return i;
> >  }
> > --
> > 2.17.2
> >