4:39 p.m.
On Thu, May 31, 2018 at 8:31 AM, Mikulas Patocka <mpatocka(a)redhat.com> wrote:
On Thu, 31 May 2018, Dan Williams wrote:
> On Thu, May 31, 2018 at 1:19 AM, Mikulas Patocka <mpatocka(a)redhat.com> wrote:
> >
> >
> > On Wed, 30 May 2018, Dan Williams wrote:
> >
> >> > Great find! Thanks for the due diligence. Feel free to add:
> >> >
> >> > Acked-by: Dan Williams <dan.j.williams(a)intel.com>
> >> >
> >> > ...on the reworks to unify ARM and x86.
> >>
> >> One more note. The side effect of not using dax_flush() is that you
> >> may end up flushing caches on systems where the platform has asserted
> >> it will take responsibility for flushing caches at power loss. If /
> >> when those systems become more prevalent we may want to think of a way
> >> to combine the non-temporal optimization and the cache-flush-bypass
> >> optimizations. However that is something that can wait for a later
> >> change beyond 4.18.
> >
> > We could define memcpy_flushpmem, that falls back to memcpy or
> > memcpy_flushcache, depending on whether the platform flushes the caches at
> > power loss or not.
>
> The problem is that some platforms only power fail protect a subset of
> the physical address range,
How can this be? A psysical address may be cached on any CPU, so either
there is enough power to flush all the CPUs' caches or there isn't.
How does the CPU design that protects only a part of physical addresses
look like?
It's not necessarily a CPU problem, it may be a problem of having
enough stored energy to potentially software loops to flush caches.
There's also the consideration that a general purpose platform may mix
persistent memory technologies from different vendors where some might
be flash-backed DRAM and some might be persistent media directly.
For now I don't think we need to worry about it, but I don't want to
make the assumption that this property is platform global given the
history of how persistent memory has been deployed to date.