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SubjectRE: [PATCH v5 1/5] x86/asm: Carve out a generic movdir64b() helper for general usage
Date
From: Dave Jiang
> Sent: 24 September 2020 00:11
>
> The MOVDIR64B instruction can be used by other wrapper instructions. Move
> the asm code to special_insns.h and have iosubmit_cmds512() call the
> asm function.
>
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> Reviewed-by: Tony Luck <tony.luck@intel.com>
> ---
> arch/x86/include/asm/io.h | 17 +++--------------
> arch/x86/include/asm/special_insns.h | 19 +++++++++++++++++++
> 2 files changed, 22 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
> index e1aa17a468a8..d726459d08e5 100644
> --- a/arch/x86/include/asm/io.h
> +++ b/arch/x86/include/asm/io.h
...
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 59a3e13204c3..2a5abd27bb86 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -234,6 +234,25 @@ static inline void clwb(volatile void *__p)
>
> #define nop() asm volatile ("nop")
>
> +/* The dst parameter must be 64-bytes aligned */
> +static inline void movdir64b(void *dst, const void *src)
> +{
> + /*
> + * Note that this isn't an "on-stack copy", just definition of "dst"
> + * as a pointer to 64-bytes of stuff that is going to be overwritten.
> + * In the MOVDIR64B case that may be needed as you can use the
> + * MOVDIR64B instruction to copy arbitrary memory around. This trick
> + * lets the compiler know how much gets clobbered.
> + */
> + volatile struct { char _[64]; } *__dst = dst;
> +
> + /* MOVDIR64B [rdx], rax */
> + asm volatile(".byte 0x66, 0x0f, 0x38, 0xf8, 0x02"
> + :
> + : "m" (*(struct { char _[64];} **)src), "a" (__dst)
> + : "memory");
> +}
> +
> #endif /* __KERNEL__ */

You've lost the "d" (src).
You don't need the 'memory' clobber, just:

static inline void movdir64b(void *dst, const void *src)
{
/*
* 64 bytes from dst are marked as modified for completeness.
* Since the writes bypass the cache later reads may return
* old data anyway.
*/
/* MOVDIR64B [rdx], rax */
asm volatile (".byte 0x66, 0x0f, 0x38, 0xf8, 0x02"
: "=m" ((struct { char _[64];} *)dst),
: "m" ((struct { char _[64];} *)src), "d" (src), "a" (dst));
}

I've checked that the "m" constraint on src does force (at least one
version of) gcc to actually write to the supplied buffer.

David

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