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

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Date:   Tue, 14 Nov 2023 06:02:01 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Li, Xin3" <xin3.li@...el.com>
Cc:     "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
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        "nik.borisov@...e.com" <nik.borisov@...e.com>
Subject: Re: [PATCH v12 01/37] x86/cpufeatures: Add the cpu feature bit for
 WRMSRNS

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.

https://people.kernel.org/tglx/notes-about-netiquette