linux-kernel - Re: [PATCH 2/4] perf/x86/ibs: Use CPUID region helper

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Date: Mon, 22 Apr 2024 22:41:46 +0200
From: Borislav Petkov <bp@...en8.de>
To: Robert Richter <rrichter@....com>
Cc: Dave Hansen <dave.hansen@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	linux-kernel@...r.kernel.org, jgross@...e.com, tglx@...utronix.de,
	x86@...nel.org, Kim Phillips <kim.phillips@....com>,
	Robert Richter <rric@...nel.org>
Subject: Re: [PATCH 2/4] perf/x86/ibs: Use CPUID region helper

On Mon, Apr 22, 2024 at 10:09:57PM +0200, Robert Richter wrote:
> Since we check get_cpuid_region_leaf()'s return code here we know if
> the cpud leaf exists and return IBS_CAPS_DEFAULT if not. That would
> not change the refB behaviour.

Yes.

> Though I think that case is rare or even not existing, I would just
> keep the implementation like that and as it was for for years.

Yes.

> > > This slightly modifies the functionality so that 0 is return if
> > > !IBS_CAPS_AVAIL (instead of IBS_CAPS_DEFAULT).
> > 
> > If !IBS_CAPS_AVAIL, then this is revB. But then you want to return
> > IBS_CAPS_DEFAULT there.
> 
> No, on a rebB get_cpuid_region_leaf() would be false, meaning the
> cpuid leaf is missing, function returns with IBS_CAPS_DEFAULT then.

So what functionality modification do you mean then?

When will IBS_CAPS_AVAIL be not set?

GH BKDG says about that bit:

"IBSFFV. IBS feature flags valid. Revision B = 0. Revision C = 1."

so that has been set ever since on >= revC.

And on revB we'll return IBS_CAPS_DEFAULT which has IBS_CAPS_AVAIL.

IOW, I don't see how we'll return 0 if !IBS_CAPS_AVAIL because latter
doesn't happen practically with that flow.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette