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

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Message-ID: <1515440467.7317.43.camel@infradead.org>
Date:   Mon, 08 Jan 2018 19:41:07 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     jmattson@...gle.com, aliguori@...zon.com, thomas.lendacky@....com,
        bp@...en8.de
Subject: Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and
 MSR_IA32_PRED_CMD down to the guest

On Mon, 2018-01-08 at 19:08 +0100, Paolo Bonzini wrote:
> 
> +       if (have_spec_ctrl && vmx->spec_ctrl != 0)
> +               wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
> +

I think this one probably *is* safe even without an 'else lfence',
which means that the CPU can speculate around it, but it wants a
comment explaining that someone has properly analysed it and saying
precisely why.
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