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

[Kirill A. Shutemov]

On Fri, Aug 02, 2019 at 10:15:33AM +0200, Peter Zijlstra wrote:
> 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.

Good question. I'll ask to clarify this.

-- 
 Kirill A. Shutemov