[RESEND 2/3] powerpc/memcpy: Add memcpy_mcsafe for pmem

[Jeff Moyer]

Nicholas Piggin <npiggin at gmail.com> writes:

> On Thu, 5 Apr 2018 15:53:07 +1000
> Balbir Singh <bsingharora at gmail.com> wrote:
>> I'm thinking about it, I wonder what "bytes remaining" mean in pmem context
>> in the context of a machine check exception. Also, do we want to be byte
>> accurate or cache-line accurate for the bytes remaining? The former is much
>> easier than the latter :)
>
> The ideal would be a linear measure of how much of your copy reached
> (or can reach) non-volatile storage with nothing further copied. You
> may have to allow for some relaxing of the semantics depending on
> what the architecture can support.

I think you've got that backwards.  memcpy_mcsafe is used to copy *from*
persistent memory.  The idea is to catch errors when reading pmem, not
writing to it.

> What's the problem with just counting bytes copied like usercopy --
> why is that harder than cacheline accuracy?

He said the former (i.e. bytes) is easier.  So, I think you're on the
same page.  :)

Cheers,
Jeff