linux-kernel - Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED

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Date:   Mon, 8 Jan 2018 17:32:24 -0500 (EST)
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>, aliguori@...zon.com,
        Tom Lendacky <thomas.lendacky@....com>, dwmw@...zon.co.uk,
        bp@...en8.de
Subject: Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and
 MSR_IA32_PRED_CMD down to the guest


> I have:
> 
> if (!have_spec_ctrl ||
>     (!msr_info->host_initiated &&
>      !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
> return 1;
> msr_info->data = to_vmx(vcpu)->msr_ia32_spec_ctrl;
> break;

> I have:
> 
> if (!have_spec_ctrl ||
>     (!msr_info->host_initiated &&
>      !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
> return 1;
> to_vmx(vcpu)->msr_ia32_spec_ctrl = data;
> break;

Both better than mine.

> > +       vmx_disable_intercept_for_msr(MSR_IA32_SPEC_CTRL, false);
> > +       vmx_disable_intercept_for_msr(MSR_IA32_PRED_CMD, false);
> 
> I have a lot of changes to MSR permission bitmap handling, but these
> intercepts should only be disabled when guest_cpuid_has(vcpu,
> X86_FEATURE_SPEC_CTRL).

That's harder to backport and not strictly necessary (just like
e.g. we don't check guest CPUID bits before emulation).  I agree that
your version is better, but I think the above is fine as a minimal
fix.

> Here, I have:
> 
> /*
> * If the guest is allowed to write to MSR_IA32_SPEC_CTRL,
> * store it on VM-exit.
> */
> if (guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
> add_autostore_msr(vmx, MSR_IA32_SPEC_CTRL);
> else
> clear_autostore_msr(vmx, MSR_IA32_SPEC_CTRL);
> 
> /*
> * If the guest's IA32_SPEC_CTRL MSR doesn't match the host's
> * IA32_SPEC_CTRL MSR, then add the MSR to the atomic switch
> * MSRs, so that the guest value will be loaded on VM-entry
> * and the host value will be loaded on VM-exit.
> */
> if (vmx->msr_ia32_spec_ctrl != spec_ctrl_enabled())
> add_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL,
>       vmx->msr_ia32_spec_ctrl,
>       spec_ctrl_enabled());
> else
> clear_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL);
>
> >         atomic_switch_perf_msrs(vmx);
> >
> >         vmx_arm_hv_timer(vcpu);
> > @@ -9707,6 +9733,12 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu
> > *vcpu)
> >  #endif
> >               );
> >
> > +       if (have_spec_ctrl) {
> > +               rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
> > +               if (vmx->spec_ctrl)
> > +                       wrmsrl(MSR_IA32_SPEC_CTRL, 0);
> > +       }
> > +
> 
> I know the VM-exit MSR load and store lists are probably slower, but
> I'm a little uncomfortable restoring the host's IA32_SPEC_CTRL MSR
> late if the guest has it clear and the host has it set.

There is no indirect branch before though, isn't it?

Paolo

> >         /* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed
> >         */
> >         if (vmx->host_debugctlmsr)
> >                 update_debugctlmsr(vmx->host_debugctlmsr);
> > --
> > 1.8.3.1
> >
> >
>