linux-kernel - Re: [PATCH] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h

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Message-ID: <20190815153447.GH15313@zn.tnic>
Date:   Thu, 15 Aug 2019 17:34:47 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Lendacky, Thomas" <Thomas.Lendacky@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>, Chen Yu <yu.c.chen@...el.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h

On Thu, Aug 15, 2019 at 01:47:24PM +0000, Lendacky, Thomas wrote:
> Sure, I can do that. Do we want to tie this into the nordrand option and
> add rdrand=off or keep that separate?

Yeah, I was looking at that this morning and I'd say keep 'em separate
because if you have to tie, you need to export functions and then
there's

	setup_clear_cpu_cap(X86_FEATURE_RDSEED);

in the nordrand callback but then F15h and F16h don't have RDSEED and
people would wonder, why clear RDSEED on AMD, blabla... so keeping them
separate saves us all that.

> I think this is a clearer indication that the action has taken place.

Yeah, but what does that bring us? You wanna know this now, while
testing. Once that whole effort is done, it is a useless printing of
info which you have in cpuinfo already.

> Not sure what you mean. We can't use the DMI stuff for this. So now, with
> the x86 family checks, if anyone adds some DMI stuff or x86 family stuff
> in the future that matches both the DMI and x86 family checks, this will
> be called more than once and so you need to copy any previous settings and
> add the new ones.

I had a suspicion that it was something like that. Ok, this is not a
big structure currently so I guess it is fine but if it keeps growing,
it would need a proper redesign like making it a list and callbacks
doing list_add_tail() for MSRs which get added. It would avoid that
kmalloc and copying which is silly. Please put a comment ontop why we're
copying.

> Except that X86_FEATURE_RDRAND isn't set anymore. I could create a new
> software feature that is set when the CPUID bit is cleared if that's
> preferred.

Nah, let's leave it like you had it.

Thx.

-- 
Regards/Gruss,
    Boris.

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