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

[David Laight]

From: Borislav Petkov
> Sent: 24 September 2020 14:08
> 
> 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));
> }

Doesn't look wrong now.
I'd still paint it a slightly different colour :-)

	David

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