Re: [PATCH] x86/srso: Correct the mitigation status when SMT is disabled

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Tue, Aug 15, 2023 at 10:17:53PM +0200, Borislav Petkov wrote:
> On Tue, Aug 15, 2023 at 12:58:31PM -0700, Josh Poimboeuf wrote:
> > AFAICT, nowhere in the spec does it say the SRSO_NO bit won't get set by
> > future (fixed) HW.  In fact I'd expect it will, similar to other *_NO
> > flags.
> 
> I'm pretty sure it won't.
> 
> SRSO_NO is synthesized by the hypervisor *software*. Nothing else.

Citation needed.

> It is there so that you don't check microcode version in the guest which
> is nearly impossible anyway.
> 
> > Regardless, here SRSO_NO seems to mean two different things: "reported
> > safe by host (or HW)" and "not reported safe on Zen1/2 with SMT not
> > possible".
> 
> Huh?

Can you clarify what doesn't make sense?

> > Also, in this code, the SRSO_NO+SMT combo doesn't seem logically
> > possible, as srso_show_state() only gets called if X86_BUG_SRSO is set,
> > which only happens if SRSO_NO is not set by the HW/host in the first
> > place.  So here, if boot_cpu_has(X86_FEATURE_SRSO_NO), it means SRSO_NO
> > was manually set by srso_select_mitigation(), and SMT can't possibly be
> > enabled.
> 
> Have you considered the case where Linux would set SRSO_NO when booting
> on future hardware, which is fixed?
> 
> There SRSO_NO and SMT will very much be possible.

How is that relevant to my comment?  The bug bit still wouldn't get set
and srso_show_state() still wouldn't be called.

-- 
Josh