linux-kernel - Re: [PATCH] x86/cpufeatures: Cleanup AMD speculation feature bits

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Message-ID: <20180126221026.hc2hk23zsqbqhkif@pd.tnic>
Date:   Fri, 26 Jan 2018 23:10:26 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Tom Lendacky <thomas.lendacky@....com>, x86-ml <x86@...nel.org>,
        linux-tip-commits@...r.kernel.org, hpa@...or.com,
        gregkh@...uxfoundation.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...nel.org
Subject: Re: [PATCH] x86/cpufeatures: Cleanup AMD speculation feature bits

On Fri, Jan 26, 2018 at 09:59:44PM +0000, David Woodhouse wrote:
> If we wanted to do this kind of thing, we'd do it the other way round.
> Turn the *Intel* feature into both 'IBRS' and 'IBPB' CPU-visible
> features, and have those defined in the AMD word.

You lost me here: have those defined in the AMD word?

> Then use virtual bits with "" for the software features, since we
> don't want *those* to appear in /proc/cpuinfo.

Whatever we do, I think it would be most consistent to have three
strings, *both* on Intel and AMD visible in cpuinfo: "ibrs", "ibpb" and
"stibp" so that there's no confusion what is enabled on each box.

Now, those three can be the *virtual* features which get set by the
actual CPUID features on init. And the latter, the *actual* CPUID
features don't need to be visible in cpuinfo: people shouldn't care
whether "spec_ctrl" on Intel and "pred_cmd" on AMD both mean "ibpb". It
should be simply "ibpb" on both vendors in cpuinfo.

Ditto for the others.

This way you have one unified message of what is enabled on *any* box.

-- 
Regards/Gruss,
    Boris.

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