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

[Balbir Singh]

On Fri, Apr 6, 2018 at 11:26 AM, Nicholas Piggin <npiggin at gmail.com> wrote:
> On Thu, 05 Apr 2018 16:40:26 -0400
> Jeff Moyer <jmoyer at redhat.com> wrote:
>
>> 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.
>>

I know the comment in x86 says posted writes and cares for only loads, but I
don't see why both sides should not be handled.

>> > 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.  :)
>
> Oh well that makes a lot more sense in my mind now, thanks :)

I thought the cache-aligned might make sense, since usually we'd expect the
failure to be at a cache-line level, but our copy_tofrom_user does accurate
accounting

Balbir Singh.