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

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Message-ID: <20190801203614.GA16228@zn.tnic>
Date:   Thu, 1 Aug 2019 22:36:14 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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, x86@...nel.org
Subject: Re: [PATCH] x86/asm: Add support for MOVDIR64B instruction

On Thu, Aug 01, 2019 at 11:28:08PM +0300, Kirill A. Shutemov 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));
> > }
> > 
> > Or did you have something else in mind?
> 
> Or should we add "memory" clobber instead, like we do for string
> operations?

I think Tony's in the right direction. We already do dst "sizing" like
that for the compiler in clwb().

-- 
Regards/Gruss,
    Boris.

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