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)
and
...(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));
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
