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

[Nicholas Piggin]

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.
> 
> > 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 :)