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

From: Borislav Petkov
Date: Mon Feb 04 2019 - 16:40:55 EST

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_.


> 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.


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.


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