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SubjectRe: [PATCH v5 1/5] x86/asm: Carve out a generic movdir64b() helper for general usage
On Wed, Sep 23, 2020 at 04:10:43PM -0700, Dave Jiang wrote:
> +/* 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");
> +}

Ok, Micha and I hashed it out on IRC, here's what you do. Please keep
the comments too because we will forget soon again.

static inline void movdir64b(void *__dst, const void *src)
{
struct { char _[64]; } *__src = src;
struct { char _[64]; } *__dst = dst;

/*
* MOVDIR64B %(rdx), rax.
*
* Both __src and __dst must be memory constraints in order to tell the
* compiler that no other memory accesses should be reordered around
* this one.
*
* Also, both must be supplied as lvalues because this tells
* the compiler what the object is (its size) the instruction accesses.
* I.e., not the pointers but what they point, thus the deref'ing '*'.
*/
asm volatile(".byte 0x66, 0x0f, 0x38, 0xf8, 0x02"
: "+m" (*__dst)
: "m" (*__src), "a" (__dst), "d" (__src));
}

Thx.

--
Regards/Gruss,
Boris.

https://people.kernel.org/tglx/notes-about-netiquette

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