linux-kernel - Re: x86/idle: Remove barriers for X86_BUG_CLFLUSH

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Message-ID: <eeca95c5-5724-4205-8449-644d91eafd48@intel.com>
Date: Wed, 2 Apr 2025 07:01:27 -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: x86/idle: Remove barriers for X86_BUG_CLFLUSH_MONITOR

On 4/2/25 02:10, 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.

There's a near copy-and-paste of that code here:

> static __cpuidle void mwait_idle(void)
> {
>         if (!current_set_polling_and_test()) {
>                 if (this_cpu_has(X86_BUG_CLFLUSH_MONITOR)) {
>                         mb(); /* quirk */
>                         clflush((void *)&current_thread_info()->flags);
>                         mb(); /* quirk */
>                 }

Any reason it can't get the same treatment?