linux-kernel - Re: [PATCH 1/6] x86: Add support for the pcommit instruction

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Date:	Fri, 14 Nov 2014 13:09:02 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	H Peter Anvin <h.peter.anvin@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Airlie <airlied@...ux.ie>,
	DRI <dri-devel@...ts.freedesktop.org>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 1/6] x86: Add support for the pcommit instruction

On Fri, Nov 14, 2014 at 1:07 PM, Ross Zwisler
<ross.zwisler@...ux.intel.com> wrote:
> On Wed, 2014-11-12 at 19:25 -0800, Andy Lutomirski wrote:
>> On 11/11/2014 10:43 AM, Ross Zwisler wrote:
>> > Add support for the new pcommit instruction.  This instruction was
>> > announced in the document "Intel Architecture Instruction Set Extensions
>> > Programming Reference" with reference number 319433-022.
>> >
>> > https://software.intel.com/sites/default/files/managed/0d/53/319433-022.pdf
>> >
>> > Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
>> > Cc: H Peter Anvin <h.peter.anvin@...el.com>
>> > Cc: Ingo Molnar <mingo@...nel.org>
>> > Cc: Thomas Gleixner <tglx@...utronix.de>
>> > Cc: David Airlie <airlied@...ux.ie>
>> > Cc: dri-devel@...ts.freedesktop.org
>> > Cc: x86@...nel.org
>> > ---
>> >  arch/x86/include/asm/cpufeature.h    | 1 +
>> >  arch/x86/include/asm/special_insns.h | 6 ++++++
>> >  2 files changed, 7 insertions(+)
>> >
>> > diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
>> > index 0bb1335..b3e6b89 100644
>> > --- a/arch/x86/include/asm/cpufeature.h
>> > +++ b/arch/x86/include/asm/cpufeature.h
>> > @@ -225,6 +225,7 @@
>> >  #define X86_FEATURE_RDSEED ( 9*32+18) /* The RDSEED instruction */
>> >  #define X86_FEATURE_ADX            ( 9*32+19) /* The ADCX and ADOX instructions */
>> >  #define X86_FEATURE_SMAP   ( 9*32+20) /* Supervisor Mode Access Prevention */
>> > +#define X86_FEATURE_PCOMMIT        ( 9*32+22) /* PCOMMIT instruction */
>> >  #define X86_FEATURE_CLFLUSHOPT     ( 9*32+23) /* CLFLUSHOPT instruction */
>> >  #define X86_FEATURE_AVX512PF       ( 9*32+26) /* AVX-512 Prefetch */
>> >  #define X86_FEATURE_AVX512ER       ( 9*32+27) /* AVX-512 Exponential and Reciprocal */
>> > diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
>> > index e820c08..1709a2e 100644
>> > --- a/arch/x86/include/asm/special_insns.h
>> > +++ b/arch/x86/include/asm/special_insns.h
>> > @@ -199,6 +199,12 @@ static inline void clflushopt(volatile void *__p)
>> >                    "+m" (*(volatile char __force *)__p));
>> >  }
>> >
>> > +static inline void pcommit(void)
>> > +{
>> > +   alternative(ASM_NOP4, ".byte 0x66, 0x0f, 0xae, 0xf8",
>> > +               X86_FEATURE_PCOMMIT);
>> > +}
>> > +
>>
>> Should this patch add the feature bit and cpuinfo entry to go with it?
>>
>> --Andy
>
> I think this patch does everything we need?  The text for cpuinfo is
> auto-generated in arch/x86/kernel/cpu/capflags.c from the flags defined
> in arch/x86/include/asm/cpufeature.h, I think.  Here's what I get in
> cpuinfo on my system with a faked-out CPUID saying that clwb and pcommit
> are present:
>
> $ grep 'flags' /proc/cpuinfo
> flags           : fpu <snip> erms pcommit clflushopt clwb xsaveopt
>
> The X86_FEATURE_CLWB and X86_FEATURE_PCOMMIT flags are being set up
> according to what's in CPUID, and the proper alternatives are being
> triggered.  I stuck some debug code in the alternatives code to see what
> was being patched in the presence and absence of each of the flags.
>
> Is there something else I'm missing?

No.  I just missed the magical auto-generation part.

--Andy

>
> Thanks,
> - Ross
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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