Date: Mon, 8 Jan 2018 17:33:38 -0500 (EST)
From: Paolo Bonzini <pbonzini@redhat.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        jmattson@google.com, aliguori@amazon.com,
        thomas lendacky <thomas.lendacky@amd.com>, bp@alien8.de
Message-ID: <1441531276.31733151.1515450818280.JavaMail.zimbra@redhat.com>
In-Reply-To: <1515440467.7317.43.camel@infradead.org>
References: <1515434925-10250-1-git-send-email-pbonzini@redhat.com> <1515434925-10250-4-git-send-email-pbonzini@redhat.com> <1515440467.7317.43.camel@infradead.org>
Subject: Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and
 MSR_IA32_PRED_CMD down to the guest
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Thread-Topic: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to the guest
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----- 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