Re: [PATCH 3/6] x86: Add support for the clwb instruction

Borislav Petkov
2014-11-11T19:12:47Z

On Tue, Nov 11, 2014 at 11:43:13AM -0700, Ross Zwisler wrote:
> Add support for the new clwb 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
>
> Here are some things of note:
>
> - As with the clflushopt patches before this, I'm assuming that the addressing
> mode generated by the original clflush instruction will match the new
> clflush instruction with the 0x66 prefix for clflushopt, and for the
> xsaveopt instruction with the 0x66 prefix for clwb. For all the test cases
> that I've come up with and for the new clwb code generated by this patch
> series, this has proven to be true on my test machine.
>
> - According to the SDM, xsaveopt has a form where it has a REX.W prefix. I
> believe that this prefix will not be generated by gcc in x86_64 kernel code.
> Based on this, I don't believe I need to account for this extra prefix when
> dealing with the assembly language created for clwb. Please correct me if
> I'm wrong.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> Cc: H Peter Anvin <h.peter.anvin@intel.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: David Airlie <airlied@linux.ie>
> Cc: dri-devel@lists.freedesktop.org
> Cc: x86@kernel.org
> ---
> arch/x86/include/asm/cpufeature.h | 1 +
> arch/x86/include/asm/special_insns.h | 10 ++++++++++
> 2 files changed, 11 insertions(+)
>
> diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
> index b3e6b89..fbbed34 100644
> --- a/arch/x86/include/asm/cpufeature.h
> +++ b/arch/x86/include/asm/cpufeature.h
> @@ -227,6 +227,7 @@
> #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_CLWB ( 9*32+24) /* CLWB instruction */
> #define X86_FEATURE_AVX512PF ( 9*32+26) /* AVX-512 Prefetch */
> #define X86_FEATURE_AVX512ER ( 9*32+27) /* AVX-512 Exponential and Reciprocal */
> #define X86_FEATURE_AVX512CD ( 9*32+28) /* AVX-512 Conflict Detection */
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 1709a2e..a328460 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -199,6 +199,16 @@ static inline void clflushopt(volatile void *__p)
> "+m" (*(volatile char __force *)__p));
> }
>
> +static inline void clwb(volatile void *__p)
> +{
> + alternative_io_2(".byte " __stringify(NOP_DS_PREFIX) "; clflush %P0",

Any particular reason for using 0x3e as a prefix to have the insns be
the same size or is it simply because CLFLUSH can stomach it?

:-)

> + ".byte 0x66; clflush %P0",
> + X86_FEATURE_CLFLUSHOPT,
> + ".byte 0x66; xsaveopt %P0",

Huh, XSAVEOPT?!? Shouldn't that be CLWB??

> + X86_FEATURE_CLWB,
> + "+m" (*(volatile char __force *)__p));
> +}
> +
> static inline void pcommit(void)
> {
> alternative(ASM_NOP4, ".byte 0x66, 0x0f, 0xae, 0xf8",
> --
> 1.9.3
>
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--
Regards/Gruss,
Boris.

Sent from a fat crate under my desk. Formatting is fine.
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