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SubjectRe: [PATCH v5 1/5] x86/asm: Carve out a generic movdir64b() helper for general usage
On Thu, Sep 24, 2020 at 08:24:46AM +0000, David Laight wrote:
> 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));

Now since you're so generous with your advice on random threads, please
explain what you're advising here?

The destination operand - in this case in %rax - is "destination memory
address specified as offset to ES segment in the register operand."

So what is the difference between:

...(void *dst, ... )

volatile struct { char _[64]; } *__dst = dst;


: "=m" (__dst)
: "a" (__dst)


...(void *dst, ... )


: "=m" ((struct { char _[64];} *)dst)
: "a" (__dst)

and why?

Point me to the gcc documentation where this is explained.

To cut to the chase, I don't think you need to do that, otherwise clwb()
would be broken too but perhaps you know something I don't.

Looking at clwb(), I believe the proper specification should be:

volatile struct { char _[64]; } *__dst = dst;


: "+m" (__dst)
: "a" (__dst)

And if anything, the source specification should be something like that:

volatile struct { char x[64]; } *__src = src;


"d" (__src)

because this tells gcc that the source operand would read 64 bytes
through the pointer in the %rdx reg.

So this ends up close to what you're saying but it is using local
variables to make the asm actually readable.

Lemme add Micha to Cc for sanity-checking:

Micha, the instruction is:

MOVDIR64B %(rdx), rax

"Move 64-bytes as direct-store with guaranteed 64-byte write atomicity
from the source memory operand address to destination memory address
specified as offset to ES segment in the register operand."

Do I need to tell gcc that both operands are referencing 64 bytes,
source operand is a memory reference, destination operand is an address
specified in a register?

What we have currently is:

volatile struct { char _[64]; } *dst = __dst;

/* MOVDIR64B [rdx], rax */
asm volatile(".byte 0x66, 0x0f, 0x38, 0xf8, 0x02"
: "=m" (dst)
: "d" (from), "a" (dst));



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