linux-kernel - Re: [PATCH v3 08/10] x86/setcpuid: Add kernel option setcpuid

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Message-ID: <20190204214044.GN29639@zn.tnic>
Date:   Mon, 4 Feb 2019 22:40:45 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michael Chan <michael.chan@...adcom.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v3 08/10] x86/setcpuid: Add kernel option setcpuid

On Mon, Feb 04, 2019 at 12:46:30PM -0800, Dave Hansen wrote:
> Intel can obviously add or remove enumeration for a feature after
> silicon ships.  But, that eats up microcode "patch" space which is an
> even more valuable resource than the microcode "ROM" space.  That patch
> space is a very constrained resource when creating things like the
> side-channel mitigations.  The way I read this situation is that this
> feature fills a bit small of a niche to justify consuming patch space.

Yap, makes sense. I've heard that argumentation before, btw.

> So, the compromise we reached in this case is that Intel will fully
> document the future silicon architecture, and then write the kernel
> implementation to _that_.

Yap.

> Then, for the weirdo deployments where this feature is not enumerated,
> we have the setcpuid= to fake the enumeration in software.
>
> The reason I'm pushing for setcpuid= instead of a one-off is that I
> don't expect this to be the last time Intel does this. I'd rather have
> one setcpuid= than a hundred things like "ac_split_lock_disable".

So my only issue with this is the user having to type this in in order
to get the feature.

VS

automatically enabling it during boot in early_init_intel() or so. No
need for any user intervention. It'll be just like a forgotten CPUID bit
and we've done those before.

The disable chicken bits you have for all those features which are
enumerated in CPUID anyway so there'll be no difference.

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.