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

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Date:   Mon, 8 Jan 2018 17:33:38 -0500 (EST)
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        jmattson@...gle.com, aliguori@...zon.com,
        thomas lendacky <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



----- Original Message -----
> 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" <thomas.lendacky@....com>, bp@...en8.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