Re: [PATCH v2 1/3] introduce memcpy_nocache()

[Dan Williams]

On Thu, Dec 29, 2016 at 7:52 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Dec 29, 2016 at 10:23:15AM -0800, Dan Williams wrote:
>
>> > BTW, your "it's iovec, only non-temporal stores there" logics in
>> > arch_copy_from_iter_pmem() is simply wrong - for one thing, unaligned
>> > copies will have parts done via normal stores, for another 32bit will
>> > _not_ go for non-caching codepath for short copies.  What semantics do
>> > we really need there?
>>
>> For typical pmem platforms we need to make sure all the writes are on
>> the way to memory such than a later sfence can guarantee that all
>> previous writes are visible to the platform "ADR" logic.  ADR handles
>> flushing memory controller write buffers to media. At a minimum
>> arch_copy_from_iter_pmem() needs to trigger a clwb (unordered cache
>> line writeback) of each touched cache line if it is not using a cache
>> bypassing store.
>
> Um...  Then we do have a problem - nocache variant of uaccess primitives
> does *not* guarantee that clwb is redundant.
>
> What about the requirements of e.g. tcp_sendmsg() with its use of
> skb_add_data_nocache()?  What warranties do we need there?

Yes, we need to distinguish the existing "nocache" that tries to avoid
unnecessary cache pollution and this new "must write through" semantic
for writing to persistent memory. I suspect usages of
skb_add_data_nocache() are ok since they are in the transmit path.
Receiving directly into a buffer that is expected to be persisted
immediately is where we would need to be careful, but that is already
backstopped by dirty cacheline tracking. So as far as I can see, we
should only need a new memcpy_writethrough() (?) for the pmem
direct-i/o path at present.