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

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Date:   Fri, 2 Aug 2019 10:15:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org,
        jing.lin@...el.com, bp@...en8.de, x86@...nel.org
Subject: Re: [PATCH] x86/asm: Add support for MOVDIR64B instruction

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.