linux-kernel - Re: [PATCH v2] x86: add workaround monitor bug

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Date:	Mon, 18 Jul 2016 14:49:56 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 Kernel <x86@...nel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH v2] x86: add workaround monitor bug

On Mon, Jul 18, 2016 at 2:41 PM, Jacob Pan
<jacob.jun.pan@...ux.intel.com> wrote:
> From: Peter Zijlstra <peterz@...radead.org>
>
> Monitored cached line may not wake up from mwait on certain
> Goldmont based CPUs. This patch will avoid calling
> current_set_polling_and_test() and thereby not set the TIF_ flag.
> The result is that we'll always send IPIs for wakeups.
>
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> ---
>  arch/x86/include/asm/cpufeatures.h | 2 +-
>  arch/x86/include/asm/mwait.h       | 2 +-
>  arch/x86/kernel/cpu/intel.c        | 5 +++++
>  arch/x86/kernel/process.c          | 2 +-
>  4 files changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index c64b1e9..c5097a7 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -310,5 +310,5 @@
>  #endif
>  #define X86_BUG_NULL_SEG       X86_BUG(10) /* Nulling a selector preserves the base */
>  #define X86_BUG_SWAPGS_FENCE   X86_BUG(11) /* SWAPGS without input dep on GS */
> -
> +#define X86_BUG_MONITOR                X86_BUG(12) /* IPI required to wake up remote cpu */
>  #endif /* _ASM_X86_CPUFEATURES_H */
> diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
> index 0deeb2d..f37f2d8 100644
> --- a/arch/x86/include/asm/mwait.h
> +++ b/arch/x86/include/asm/mwait.h
> @@ -97,7 +97,7 @@ static inline void __sti_mwait(unsigned long eax, unsigned long ecx)
>   */
>  static inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx)
>  {
> -       if (!current_set_polling_and_test()) {
> +       if (static_cpu_has_bug(X86_BUG_MONITOR) || !current_set_polling_and_test()) {
>                 if (static_cpu_has_bug(X86_BUG_CLFLUSH_MONITOR)) {
>                         mb();
>                         clflush((void *)&current_thread_info()->flags);
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index f380b61..fcd484d 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -13,6 +13,7 @@
>  #include <asm/msr.h>
>  #include <asm/bugs.h>
>  #include <asm/cpu.h>
> +#include <asm/intel-family.h>
>
>  #ifdef CONFIG_X86_64
>  #include <linux/topology.h>
> @@ -508,6 +509,10 @@ static void init_intel(struct cpuinfo_x86 *c)
>             (c->x86_model == 29 || c->x86_model == 46 || c->x86_model == 47))
>                 set_cpu_bug(c, X86_BUG_CLFLUSH_MONITOR);
>
> +       if (c->x86 == 6 && boot_cpu_has(X86_FEATURE_MWAIT) &&
> +               ((c->x86_model == INTEL_FAM6_ATOM_GOLDMONT)))
> +               set_cpu_bug(c, X86_BUG_MONITOR);
> +
>  #ifdef CONFIG_X86_64
>         if (c->x86 == 15)
>                 c->x86_cache_alignment = c->x86_clflush_size * 2;
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 61b703c..62c0b0e 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -405,7 +405,7 @@ static int prefer_mwait_c1_over_halt(const struct cpuinfo_x86 *c)
>         if (c->x86_vendor != X86_VENDOR_INTEL)
>                 return 0;
>
> -       if (!cpu_has(c, X86_FEATURE_MWAIT))
> +       if (!cpu_has(c, X86_FEATURE_MWAIT) || static_cpu_has_bug(X86_BUG_MONITOR))
>                 return 0;
>
>         return 1;
> --
> 2.7.4
>

It would be simpler to just force clear X86_FEATURE_MWAIT.

--
Brian Gerst