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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