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

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Message-ID: <67274a07-eac9-4d60-9737-c730c25a9fea@intel.com>
Date: Wed, 2 Apr 2025 10:35:03 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>,
 LKML <linux-kernel@...r.kernel.org>
Cc: 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

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.

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