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

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Message-ID: <Z_K4HzutEoEQFSco@gmail.com>
Date: Sun, 6 Apr 2025 19:21:35 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Uros Bizjak <ubizjak@...il.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


* Uros Bizjak <ubizjak@...il.com> wrote:

> There is another instance of the same sequence in arch/x86/kernel/smpboot.c:
> 
> 		/*
> 		 * The CLFLUSH is a workaround for erratum AAI65 for
> 		 * the Xeon 7400 series.  It's not clear it is actually
> 		 * needed, but it should be harmless in either case.
> 		 * The WBINVD is insufficient due to the spurious-wakeup
> 		 * case where we return around the loop.
> 		 */
> 		mb();
> 		clflush(md);
> 		mb();
> 		__monitor(md, 0, 0);
> 		mb();
> 		__mwait(eax_hint, 0);

True, but I wouldn't touch that - the shutdown path is not a 
performance critical code path in any fashion, and it's often
difficult to debug on real hardware, so the cost/benefit factor
is abnormally high.

Thanks,

	Ingo