Re: [PATCH 38/39] x86/mm/pti: Add Warning when booting on a PCID capable CPU

[Joerg Roedel]

On Fri, Jul 13, 2018 at 11:59:44AM -0700, Andy Lutomirski wrote:
> On Wed, Jul 11, 2018 at 4:29 AM, Joerg Roedel <joro@xxxxxxxxxx> wrote:
> > From: Joerg Roedel <jroedel@xxxxxxx>
> >
> > Warn the user in case the performance can be significantly
> > improved by switching to a 64-bit kernel.
> 
> ...
> 
> > +#ifdef CONFIG_X86_32
> > +       if (boot_cpu_has(X86_FEATURE_PCID)) {
> 
> I'm a bit confused. Wouldn't the setup_clear_cpu_cap() call in
> early_identify_cpu() prevent this from working?

Right you are, I don't have a PCID capable system at hand for testing,
so I didn't catch this...

> Boris, do we have a straightforward way to ask "does the CPU advertise
> this feature in CPUID regardless of whether we have it enabled right
> now"?

I guess we need to call cpuid again.


Regards,

	Joerg