Re: [PATCH v5 1/5] x86/asm: Carve out a generic movdir64b() helper for general usage

[Borislav Petkov]

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.

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