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SubjectRe: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to the guest


----- Original Message -----
> From: "David Woodhouse" <dwmw2@infradead.org>
> To: "Paolo Bonzini" <pbonzini@redhat.com>, linux-kernel@vger.kernel.org, kvm@vger.kernel.org
> Cc: jmattson@google.com, aliguori@amazon.com, "thomas lendacky" <thomas.lendacky@amd.com>, bp@alien8.de
> Sent: Monday, January 8, 2018 8:41:07 PM
> 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.

This one is okay as long as there are no indirect jumps until
vmresume. But the one on vmexit is only okay because right now
it's *disabling* IBRS. Once IBRS is used by Linux, we'll need an
lfence there. I'll add a comment.

Paolo

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