linux-kernel - Re: [PATCH] x86/smpboot: check if CLFLUSH is actually necessary

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Message-ID: <20150130233142.GA1884@pd.tnic>
Date:	Sat, 31 Jan 2015 00:31:42 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Scotty Bauer <sbauer@....utah.edu>
Cc:	tglx@...utronix.de, mingo@...heat.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] x86/smpboot: check if CLFLUSH is actually necessary

On Fri, Jan 30, 2015 at 02:26:17PM -0700, Scotty Bauer wrote:
> mwait_play_dead previously issued a CLFLUSH to work around a bug on some xeon processors. We can now determine if the CPU is a buggy CPU. This patch checks if if we're on a buggy CPU which allows non-buggy cpu's to eliminate the CLFLUSH.
> 
> 
> 
> 
> 

> From 3da1be5c998a8d51f98fdba09b3cb477526c5ff3 Mon Sep 17 00:00:00 2001
> From: Scott Bauer <sbauer@....utah.edu>
> Date: Fri, 30 Jan 2015 14:10:37 -0700
> Subject: [PATCH] Add check to determine if CLFLUSH is actually necessary
>  before monitor/wait
> 
> Signed-off-by: Scott Bauer <sbauer@....utah.edu>
> ---
>  arch/x86/kernel/smpboot.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 6d7022c..552ff48 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1384,6 +1384,7 @@ static inline void mwait_play_dead(void)
>  	unsigned int highest_subcstate = 0;
>  	void *mwait_ptr;
>  	int i;
> +	int cpu;
>  
>  	if (!this_cpu_has(X86_FEATURE_MWAIT))
>  		return;
> @@ -1420,6 +1421,7 @@ static inline void mwait_play_dead(void)
>  	 * content is immaterial as it is not actually modified in any way.
>  	 */
>  	mwait_ptr = &current_thread_info()->flags;
> +	cpu = smp_processor_id();
>  
>  	wbinvd();
>  
> @@ -1430,10 +1432,15 @@ static inline void mwait_play_dead(void)
>  		 * 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.
> +		 *
> +		 * Check if the CLFLUSH is actually necessary before calling
>  		 */
> -		mb();
> -		clflush(mwait_ptr);
> -		mb();
> +		if (cpu_has_bug(&cpu_data(cpu), X86_BUG_CLFLUSH_MONITOR)) {
> +			mb();
> +			clflush(mwait_ptr);
> +			mb();

Or you can so something even better:

	alternative(ASM_NOP3, "clflush", X86_BUG_CLFLUSH_MONITOR);

Well, almost, you'd also need to pass in mwait_ptr, i.e. something like that:

https://lkml.kernel.org/r/1422377631-8986-3-git-send-email-ross.zwisler@linux.intel.com

but simpler. Maybe this:

	asm volatile(ALTERNATIVE(ASM_NOP3, "clflush %[p]", X86_BUG_CLFLUSH_MONITOR)
		      : [p] "+m" (*mwait_ptr));

Totally untested though - it is supposed to show the idea only.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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