linux-kernel - Re: [PATCH v2] x86/idle: Remove MFENCEs for X86_BUG_CLFLUSH

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Message-ID: <Z-2YOvIVDfttHFdq@gmail.com>
Date: Wed, 2 Apr 2025 22:04:10 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2] x86/idle: Remove MFENCEs for X86_BUG_CLFLUSH_MONITOR


* Dave Hansen <dave.hansen@...el.com> wrote:

> On 4/2/25 10:24, Andrew Cooper wrote:
> ...
> > i.e. The SDM was incorrect at the time, and barriers should not have been
> > inserted.  Double checking the original AAI65 errata (not available from
> > intel.com any more) shows no mention of barriers either.
> > 
> > Note: If this were a general codepath, the MFENCEs would be needed, because
> >       AMD CPUs of the same vintage do sport otherwise-unordered CLFLUSHs.
> > 
> > Furthermore, use a plain alternative, rather than static_cpu_has_bug() and/or
> > no optimisation.  The workaround is a single instruction.
> 
> Nit: this never comes out and says that it removes the unnecessary
> barriers. But we can fix that up when we apply this by adding:
> 
> 	Remove the unnecessary barriers. Furthermore, use a plain
> 	alternative, rather than static_cpu_has_bug() and/or
> 	no optimisation.  The workaround is a single instruction.

So the title says it already:

  x86/idle: Remove MFENCEs for X86_BUG_CLFLUSH_MONITOR

But I've added in your sentence as well, because there's no such thing 
as too much clarity in a changelog. :-)

> Acked-by: Dave Hansen <dave.hansen@...ux.intel.com>

Thanks!

	Ingo