Re: [PATCH v12 01/37] x86/cpufeatures: Add the cpu feature bit for WRMSRNS

Borislav Petkov
2023-11-14T05:02:41Z

On Tue, Nov 14, 2023 at 12:43:38AM +0000, Li, Xin3 wrote:
> No. tglx asked for it:
> https://lkml.kernel.org/kvm/87y1h81ht4.ffs@tglx/

Aha

"According to the CPU folks FRED systems are guaranteed to have WRMSRNS -
I asked for that :). It's just not yet documented."

so I'm going to expect that to appear in the next FRED spec revision...

> Because we are doing
> wrmsrns(MSR_IA32_FRED_RSP0, ...)
> here, and X86_FEATURE_WRMSRNS doesn't guarantee MSR_IA32_FRED_RSP0 exists.
>
> Or I missed something?

Well, according to what I'm hearing and reading so far:

FRED means WRMSRNS
FRED means MSR_IA32_FRED_RSP0

and if you had to be precise, the code should do:

if (cpu_feature_enabled(X86_FEATURE_FRED)) {
if (cpu_feature_enabled(X86_FEATURE_WRMSRNS))
wrmsrns(MSR_IA32_FRED_RSP0, (unsigned long)task_stack_page(task) + THREAD_SIZE);
else
wrmsr(MSR_IA32_FRED_RSP0, (unsigned long)task_stack_page(task) + THREAD_SIZE);
}

but apparently FRED implies WRMSRNS - not documented anywhere currently
- so you can save yourself one check.

But your version checks FRED if it can do WRMSRNS while there's
a separate WRMSRNS flag and that made me wonder...

> Another patch set should replace WRMSR with WRMSRNS, with SERIALIZE added
> when needed.

I sense someone wants to optimize MSR writes ... :-)

Thx.

--
Regards/Gruss,
Boris.