From: Borislav Petkov <bp@alien8.de>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Matthias Welwarsky <matthias.welwarsky@sysgo.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
linux-kernel@vger.kernel.org, x86-ml <x86@kernel.org>
Subject: Re: x86, possible bug in __memmove() alternatives patching
Date: Sat, 26 Mar 2022 09:27:55 +0100 [thread overview]
Message-ID: <Yj7Oi9g/B54xAcmF@zn.tnic> (raw)
In-Reply-To: <d9d0405e-b118-b028-d26f-fbb8de4e7a0e@intel.com>
On Fri, Mar 25, 2022 at 09:45:24PM -0700, Dave Hansen wrote:
> On 3/25/22 15:07, Borislav Petkov wrote:
> > I know it's is probably a very rare case and Intel recommends having fast
> > string ops enabled, hence the question: would this be considered a bug in the
> > kernel that should be fixed? A potential fix could be to clear FSRM together
> > with ERMS depending on IA32_MISC_ENABLE.
>
> I'd consider it a bug in the hypervisor, personally. ;)
That's a given.
>
> But, we do try to make the kernel work even the face of funky
> hypervisors that do things that never occur on real hardware. If a nice
> patch to fix this up showed up, I'd definitely take a look.
So, more to the point, it is about this chunk:
/*
* If fast string is not enabled in IA32_MISC_ENABLE for any reason,
* clear the fast string and enhanced fast string CPU capabilities.
*/
if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
if (!(misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)) {
pr_info("Disabled fast string operations\n");
setup_clear_cpu_cap(X86_FEATURE_REP_GOOD);
setup_clear_cpu_cap(X86_FEATURE_ERMS);
}
}
we already check whether fast strings was disabled, regardless of HV or
not. Question is, should we clear X86_FEATURE_FSRM there too. I wanna
say yes.
Or is it that, *if* MSR_IA32_MISC_ENABLE_FAST_STRING is clear, the FSRM
CPUID bit was not set either so nothing to clear...
Hmm?
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
next prev parent reply other threads:[~2022-03-26 8:28 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-25 8:51 x86, possible bug in __memmove() alternatives patching Matthias Welwarsky
2022-03-25 22:07 ` Borislav Petkov
2022-03-26 4:45 ` Dave Hansen
2022-03-26 8:27 ` Borislav Petkov [this message]
2022-03-29 22:34 ` Dave Hansen
2022-03-26 11:39 ` Matthias Welwarsky
2022-03-29 22:33 ` Dave Hansen
2022-03-30 13:56 ` Matthias Welwarsky
2022-03-30 14:44 ` Dave Hansen
2022-03-30 14:54 ` Borislav Petkov
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