Re: [PATCH] x86/bugs: Switch to "auto" when "ibrs" selected on Enhanced IBRS parts

[Pawan Gupta]

On Thu, Jul 14, 2022 at 10:38:14AM -0700, Josh Poimboeuf wrote:
> On Thu, Jul 14, 2022 at 07:03:32PM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 14, 2022 at 09:01:06AM -0700, Josh Poimboeuf wrote:
> >
> > > > Yeah this; if the user asks for IBRS, we should give him IBRS. I hate
> > > > the 'I know better, let me change that for you' mentality.
> > >
> > > eIBRS CPUs don't even have legacy IBRS so I don't see how this is even
> > > possible.
> >
> > You can still WRMSR a lot on them. Might not make sense but it 'works'.
>
> Even in Intel documentation, eIBRS is often referred to as IBRS. It
> wouldn't be surprising for a user to consider spectre_v2=ibrs to mean
> "use eIBRS".
>
> I'm pretty sure there's nobody out there that wants spectre_v2=ibrs to
> mean "make it slower and possibly less secure because it's being used
> contrary to the spec".

Apart from testing, I don't see a reason for a user to deliberately
choose =ibrs on Enhanced IBRS parts. But, I am guessing most users would
just rely on "=auto" mode.

So honoring what the user asked and printing a warning may be fine. And
hope they would see the warning if they unintentionally chose "=ibrs" on
an eIBRS part.

Thanks,
Pawan