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    SubjectRe: [PATCH v2] x86, uaccess: introduce copy_from_iter_wt for pmem / writethrough operations

    * Dan Williams <dan.j.williams@intel.com> wrote:

    > On Sat, May 6, 2017 at 2:46 AM, Ingo Molnar <mingo@kernel.org> wrote:
    > >
    > > * Dan Williams <dan.j.williams@intel.com> wrote:
    > >
    > >> On Fri, May 5, 2017 at 3:44 PM, Kani, Toshimitsu <toshi.kani@hpe.com> wrote:
    > >> > On Fri, 2017-05-05 at 15:25 -0700, Dan Williams wrote:
    > >> >> On Fri, May 5, 2017 at 1:39 PM, Kani, Toshimitsu <toshi.kani@hpe.com>
    > >> >> wrote:
    > >> > :
    > >> >> > > ---
    > >> >> > > Changes since the initial RFC:
    > >> >> > > * s/writethru/wt/ since we already have ioremap_wt(),
    > >> >> > > set_memory_wt(), etc. (Ingo)
    > >> >> >
    > >> >> > Sorry I should have said earlier, but I think the term "wt" is
    > >> >> > misleading. Non-temporal stores used in memcpy_wt() provide WC
    > >> >> > semantics, not WT semantics.
    > >> >>
    > >> >> The non-temporal stores do, but memcpy_wt() is using a combination of
    > >> >> non-temporal stores and explicit cache flushing.
    > >> >>
    > >> >> > How about using "nocache" as it's been
    > >> >> > used in __copy_user_nocache()?
    > >> >>
    > >> >> The difference in my mind is that the "_nocache" suffix indicates
    > >> >> opportunistic / optional cache pollution avoidance whereas "_wt"
    > >> >> strictly arranges for caches not to contain dirty data upon
    > >> >> completion of the routine. For example, non-temporal stores on older
    > >> >> x86 cpus could potentially leave dirty data in the cache, so
    > >> >> memcpy_wt on those cpus would need to use explicit cache flushing.
    > >> >
    > >> > I see. I agree that its behavior is different from the existing one
    > >> > with "_nocache". That said, I think "wt" or "write-through" generally
    > >> > means that writes allocate cachelines and keep them clean by writing to
    > >> > memory. So, subsequent reads to the destination will hit the
    > >> > cachelines. This is not the case with this interface.
    > >>
    > >> True... maybe _nocache_strict()? Or, leave it _wt() until someone
    > >> comes along and is surprised that the cache is not warm for reads
    > >> after memcpy_wt(), at which point we can ask "why not just use plain
    > >> memcpy then?", or set the page-attributes to WT.
    > >
    > > Perhaps a _nocache_flush() postfix, to signal both that it's non-temporal and that
    > > no cache line is left around afterwards (dirty or clean)?
    >
    > Yes, I think "flush" belongs in the name, and to make it easily
    > grep-able separate from _nocache we can call it _flushcache? An
    > efficient implementation will use _nocache / non-temporal stores
    > internally, but external consumers just care about the state of the
    > cache after the call.

    _flushcache() works for me too.

    Thanks,

    Ingo

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