linux-kernel - Re: [PATCH] x86: also disable FSRM if ERMS is disabled

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Date:   Tue, 11 Oct 2022 19:52:53 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Daniel Verkamp <dverkamp@...omium.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH] x86: also disable FSRM if ERMS is disabled

On Tue, Oct 11, 2022 at 05:09:19PM +0000, Luck, Tony wrote:
> >> The Intel SDM says: "Software can disable fast-string operation by
> >> clearing the fast-string-enable bit (bit 0) of IA32_MISC_ENABLE MSR",
> >> so it's not an invalid configuration for this bit to be unset.
> >
> > Dunno, did Intel folks think about clearing the respective CPUID bits
> > when exposing IA32_MISC_ENABLE[0] to software? Tony?
> 
> I don't know if it was thought about. Experimentally clearing bit
> 0 of IA32_MISC_ENABLE does not affect the CPUID bit settings
> for either ERMS or FSRM (on the one system I tried that supports
> both of these bits).

Confirms my observation on a Coffeelake here too.

Oh well, probably not that important. I mean, I bet luserspace is
looking at those CPUID bits - glibc probably - and then probably selects
the proper memcpy or so routine, based on what's there.

If something clears MSR_IA32_MISC_ENABLE_FAST_STRING_BIT and we go and
clear our feature flags and luserspace still queries CPUID then oh well,
it'll be fun. It all depends on why something has cleared them tho. It
could be some performance thing or something a lot more funky. I guess
if stuff starts exploding left and right, there will soon be a microcode
patch after that. :-)

Might be prudent to check with hw folks just in case...

Thx.

-- 
Regards/Gruss,
    Boris.

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