Re: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure

[Eduardo Habkost]

On Mon, Jan 29, 2018 at 07:32:06PM -0800, Linus Torvalds wrote:
> On Mon, Jan 29, 2018 at 5:32 PM, Arjan van de Ven <arjan@xxxxxxxxxxxxxxx> wrote:
> >
> > the most simple solution is that we set the internal feature bit in Linux
> > to turn on the "stuff the RSB" workaround is we're on a SKL *or* as a guest
> > in a VM.
> 
> That sounds reasonable.
> 
> However, wouldn't it be even better to extend on the current cpuid
> model, and actually have some real architectural bits in there.

If Intel could do that, it would be great.


> 
> Maybe it could be a bit in that IA32_ARCH_CAPABILITIES MSR. Say, add a
> bit #2 that says "ret falls back on BTB".
> 
> Then that bit basically becomes the "Skylake bit". Hmm?

Yes.  But note that the OS needs to be able to differentiate "old
Skylake that doesn't support the new bit" from "newer Skylake
that doesn't fall back om BTB".  That's why I suggest a
"non-Skylake bit" instead of a "Skylake bit".

-- 
Eduardo