Re: [PATCH RFC 01/14] x86/asm: add iosubmit_cmds512() based on movdir64b CPU instruction

[Dave Jiang]

On 11/20/19 2:50 PM, Hansen, Dave wrote:
> On 11/20/19 1:23 PM, Dave Jiang wrote:
> > +static inline void __iowrite512(void __iomem *__dst, const void *src)
> > +{
> > +	volatile struct { char _[64]; } *dst = __dst;
>
> This _looks_ like gibberish.  I know it's not, but it is subtle enough
> that it really needs specific comments.

I'll add comments explaining.

> > +static inline void iosubmit_cmds512(void __iomem *dst, const void *src,
> > +				    size_t count)
> > +{
> > +	const u8 *from = src;
> > +	const u8 *end = from + count * 64;
> > +
> > +	if (!cpu_has_write512())
> > +		return;
> > +
> > +	while (from < end) {
> > +		__iowrite512(dst, from);
> > +		from += 64;
> > +	}
> > +}
>
> Won't this silently just drop things if the CPU doesn't have movdir64b
> support?
>
> It seems like this shouldn't be called at all if
> !cpu_has_write512(), but wouldn't something like this be mroe appropriate?
>
> 	if (!cpu_has_write512()) {
> 		WARN_ON_ONCE(1);
> 		return;
> 	}
>
> Is the caller just supposed to infer that "dst" was never overwritten?

Thanks. I'll add the WARN().