RE: [PATCH v4 1/5] x86/asm: move the raw asm in iosubmit_cmds512() to special_insns.h

[David Laight <David.Laight@ACULAB.COM>]

From: Dave Jiang
> Sent: 23 September 2020 16:43
...
> >
> > On Thu, Sep 17, 2020 at 02:15:16PM -0700, Dave Jiang wrote:
> >> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> >> index 59a3e13204c3..7bc8e714f37e 100644
> >> --- a/arch/x86/include/asm/special_insns.h
> >> +++ b/arch/x86/include/asm/special_insns.h
> >> @@ -234,6 +234,23 @@ static inline void clwb(volatile void *__p)
> >>
> >>   #define nop() asm volatile ("nop")
> >>
> >> +static inline void movdir64b(void *__dst, const void *src)
> >
> > Make __dst be the function local variable name and keep "dst", i.e.,
> > without the underscores, the function parameter name.
> 
> Ok will fix
> 
> >
> >> +	/*
> >> +	 * Note that this isn't an "on-stack copy", just definition of "dst"
> >> +	 * as a pointer to 64-bytes of stuff that is going to be overwritten.
> >> +	 * In the MOVDIR64B case that may be needed as you can use the
> >> +	 * MOVDIR64B instruction to copy arbitrary memory around. This trick
> >> +	 * lets the compiler know how much gets clobbered.
> >> +	 */
> >> +	volatile struct { char _[64]; } *dst = __dst;
> >> +
> >> +	/* MOVDIR64B [rdx], rax */
> >> +	asm volatile(".byte 0x66, 0x0f, 0x38, 0xf8, 0x02"
> >> +		     : "=m" (dst)
> >> +		     : "d" (src), "a" (dst));
> >> +}

Since 'dst' needs to be 64byte aligned it isn't clear that 'void *'
is the right type for 'dst'.
At least add a comment.

Your asm constraints are also just wrong.

There is no real point specifying "=m" (dst) as an output.
The write rather bypasses the cache and the caller better
know what they are doing.

OTOH you'd better add "m" ((struct { char _[64];} *)src) as
an input constraint.

	David

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