Re: [PATCH 0/6] use memcpy_mcsafe() for copy_to_iter()

[Dan Williams <dan.j.williams@intel.com>]

On Tue, May 1, 2018 at 8:33 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Tue, May 1, 2018 at 8:22 PM Dan Williams <dan.j.williams@intel.com>
> wrote:
>
>> All that to say that having a typical RAM page covering poisoned pmem
>> would complicate the 'clear badblocks' implementation.
>
> Ugh, ok.
>
> I guess the good news is that your patches aren't so big, and don't really
> affect anything else.
>
> But can we at least take this to be the impetus for just getting rid of
> that disgusting unrolled memcpy? Ablout half of the lines in the patch set
> comes from that thing.
>
> Is anybody seriously going to use pmem with some in-order chip that can't
> even get something as simple as a memory copy loop right? "git blame"
> fingers Tony Luck, I think he may have been influenced by the fumes from
> Itanium.
>
> I  have some dim memory of "rep movs doesn't work well for pmem", but does
> it *seriously* need unrolling to cacheline boundaries? And if it does, who
> designed it, and why is anybody using it?
>

I think this is an FAQ from the original submission, in fact some guy
named "Linus Torvalds" asked [1]:

---

>  - why does this use the complex - and slower, on modern machines -
> unrolled manual memory copy, when you might as well just use a single
>
>      rep ; movsb
>
>     which not only makes it smaller, but makes the exception fixup trivial.

Because current generation cpus don't give a recoverable machine
check if we consume with a "rep ; movsb" :-(
When we have that we can pick the best copy function based
on the capabilities of the cpu we are running on.

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[1]: https://lkml.org/lkml/2016/2/18/608