Re: [PATCH] x86/asm: Add support for MOVDIR64B instruction

Peter Zijlstra
2019-08-02T08:15:45Z

On Thu, Aug 01, 2019 at 12:49:48PM -0700, Luck, Tony wrote:
> On Thu, Aug 01, 2019 at 10:43:48PM +0300, Alexey Dobriyan wrote:
> > > +static inline void movdir64b(void *dst, const void *src)
> > > +{
> > > + /* movdir64b [rdx], rax */
> > > + asm volatile(".byte 0x66, 0x0f, 0x38, 0xf8, 0x02"
> > > + : "=m" (*(char *)dst)
> > ^^^^^^^^^^
> >
> > > + : "d" (src), "a" (dst));
> > > +}
> >
> > Probably needs fake 64-byte type, so that compiler knows what is dirty.
>
> Would that be something like this?
>
> static inline void movdir64b(void *dst, const void *src)
> {
> struct dstbytes {
> char pad[64];
> };
>
> /* movdir64b [rdx], rax */
> asm volatile(".byte 0x66, 0x0f, 0x38, 0xf8, 0x02"
> : "=m" (*(struct dstbytes *)dst)
> : "d" (src), "a" (dst));
> }

Can the source and destination overlap? The SDM doesn't seem to mention
this.

Also, it bugs me something fierce that this provides a
single-copy-atomic 64b store, but there is no matching load operation.