linux-kernel - Re: [PATCH 1/4] x86/speculation: Use IBRS if available before calling into firmware

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Date:   Wed, 14 Feb 2018 16:53:39 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Tom Lendacky <thomas.lendacky@....com>, tglx@...utronix.de,
        karahmed@...zon.de, x86@...nel.org, kvm@...r.kernel.org,
        torvalds@...ux-foundation.org, pbonzini@...hat.com,
        linux-kernel@...r.kernel.org, bp@...en8.de, peterz@...radead.org,
        jmattson@...gle.com, rkrcmar@...hat.com,
        arjan.van.de.ven@...el.com, dave.hansen@...el.com, mingo@...nel.org
Subject: Re: [PATCH 1/4] x86/speculation: Use IBRS if available before
 calling into firmware



On Wed, 2018-02-14 at 10:36 -0600, Tom Lendacky wrote:
> On 2/14/2018 10:11 AM, David Woodhouse wrote:
> > 
> > 
> > 
> > On Wed, 2018-02-14 at 10:07 -0600, Tom Lendacky wrote:
> > > 
> > > Shouldn't these writes to the MSR be just for the IBRS bit?  The spec
> > > also defines the STIBP bit for this MSR, and if that bit had been set by
> > > BIOS for example, these writes will clear it.  And who knows what future
> > > bits may be defined and how they'll be used.
> >
> > We don't use STIBP. If one day we do decide to set it in userspace for
>
> Right, I understand the kernel doesn't use STIBP, that's why I mentioned
> BIOS as an example.

BIOS has no business setting this for us either. Either we support it
and turn it on in the kernel ourselves, or it's off.
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