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

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Message-ID: <20190802125818.wvyqsrcfm562bdpp@box>
Date:   Fri, 2 Aug 2019 15:58:18 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Luck, Tony" <tony.luck@...el.com>,
        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 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