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From:   Jim Mattson <jmattson@google.com>
Date:   Thu, 15 Feb 2018 20:02:15 -0800
Message-ID: <CALMp9eQMtV+f2+uYzeq2SCmvCVBAZ9G9pxJWV-BbAsSLbqR5zg@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:

> @@ -8828,6 +8890,15 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>
>         vmx_arm_hv_timer(vcpu);
>
> +       /*
> +        * If this vCPU has touched SPEC_CTRL, restore the guest's value if
> +        * it's non-zero. Since vmentry is serialising on affected CPUs, there
> +        * is no need to worry about the conditional branch over the wrmsr
> +        * being speculatively taken.
> +        */
> +       if (vmx->spec_ctrl)
> +               wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
> +

Shouldn't this be wrmsrl_safe? Userspace can make an ioctl to set
vmx->spec_ctrl to non-zero even if the MSR is not supported on the
host.