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

Borislav Petkov
2015-01-30T23:32:02Z

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.
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> From 3da1be5c998a8d51f98fdba09b3cb477526c5ff3 Mon Sep 17 00:00:00 2001
> From: Scott Bauer <sbauer@eng.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@eng.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.