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

From: Borislav Petkov
Date: Tue Feb 05 2019 - 01:18:11 EST

On Mon, Feb 04, 2019 at 03:24:23PM -0800, Dave Hansen wrote:
> Actually, there's one part of all this that I forgot. Will split lock
> detection be enumerated _widely_?

You never know what users will do. The moment it gets out, it better be
designed properly, along with the chicken bits.

> IOW, will my laptop in 5 years enumerate support for it?

Don't tell me this is going to be another MPX fiasco :-\

Or is this something along the lines of "let's see whether it takes off
and if yes, we'll commit to it or otherwise remove it and not even waste
a CPUID leaf"?

> If so, we surely don't want to enable this everyhwhere: it will break
> old apps. Doesn't that mean that we need both feature detection and
> another separate bit for folks to opt-in?

Well, if it breaks old apps, it probably needs to be opt-in anyway.

And for that you don't need setcpuid either - you simply boot with
"split_lock_ac" or whatever and the kernel pokes that MSR_TEST_CTL or
whatever else it needs to detect in hw for split lock and sets the
X86_FEATURE bits if detection is successful.


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