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From:   Jim Mattson <jmattson@google.com>
Date:   Thu, 15 Feb 2018 20:16:32 -0800
Message-ID: <CALMp9eSBqM7YoyFb+darU=1r33vR_P+Yoo7HqX=_M-wyu0zM8g@mail.gmail.com>
Subject: Re: [PATCH 8/9] KVM/VMX: Allow direct access to MSR_IA32_SPEC_CTRL
To:     David Woodhouse <dwmw@amazon.co.uk>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org
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On Tue, Feb 6, 2018 at 9:29 AM, David Woodhouse <dwmw@amazon.co.uk> wrote:

> @@ -8946,6 +9017,27 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  #endif
>               );
>
> +       /*
> +        * We do not use IBRS in the kernel. If this vCPU has used the
> +        * SPEC_CTRL MSR it may have left it on; save the value and
> +        * turn it off. This is much more efficient than blindly adding
> +        * it to the atomic save/restore list. Especially as the former
> +        * (Saving guest MSRs on vmexit) doesn't even exist in KVM.
> +        *
> +        * For non-nested case:
> +        * If the L01 MSR bitmap does not intercept the MSR, then we need to
> +        * save it.
> +        *
> +        * For nested case:
> +        * If the L02 MSR bitmap does not intercept the MSR, then we need to
> +        * save it.
> +        */
> +       if (!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL))
> +               rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
> +
> +       if (vmx->spec_ctrl)
> +               wrmsrl(MSR_IA32_SPEC_CTRL, 0);
> +

Again, we haven't verified host support for this MSR. Perhaps this
should be something like:

if (!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)
    && !rdmsrl_safe(MSR_IA32_SPEC_CTRL, &vmx->spec_ctrl) && vmx->spec_ctrl)
        wrmsrl(MSR_IA32_SPEC_CTRL, 0);