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

David Woodhouse
2018-02-14T16:53:55Z

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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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