On 2022-08-09 11:34, Morten Brørup wrote:
From: Mattias Rönnblom [mailto:hof...@lysator.liu.se]
Sent: Sunday, 7 August 2022 22.20
On 2022-07-29 22:26, Morten Brørup wrote:
+TO: @Honnappa, we need input from ARM
From: Konstantin Ananyev [mailto:konstantin.anan...@huawei.com]
Sent: Friday, 29 July 2022 21.49
From: Konstantin Ananyev [mailto:konstantin.anan...@huawei.com]
Sent: Friday, 29 July 2022 14.14
Sorry, missed that part.
Another question - who will do 'sfence' after the copying?
Would it be inside memcpy_nt (seems quite costly), or would
it be another API function for that: memcpy_nt_flush() or so?
Outside. Only the developer knows when it is required, so it
wouldn't
make any sense to add the cost inside memcpy_nt().
I don't think we should add a flush function; it would just be
another name for an already existing function. Referring to the
required
operation in the memcpy_nt() function documentation should
suffice.
Ok, but again wouldn't it be arch specific?
AFAIK for x86 it needs to boil down to sfence, for other
architectures
- I don't know.
If you think there already is some generic one (rte_wmb?) that
would
always produce
correct instructions - sure let's use it.
DPDK has generic functions to wrap architecture specific stuff like
memory barriers.
Because they are non-temporal stores, I suspect that rte_mb() is
required before reading the data from the location it was copied to.
Ensuring that STORE operations are ordered (rte_wmb) might not
suffice. However, I'm not a CPU expert, so I will seek advice from
more qualified people in the community on this.
I think for IA sfence is enough, see citation below,
for other architectures - no idea.
What I am trying to say - it needs to be the *same* function on all
archs we support.
Now I get it: rte_wmb() might be appropriate on x86, but if any other
architecture requires something else, we should add a new common
function for flushing, e.g. rte_memcpy_nt_flush().
rte_wmb() not being enough also my understanding. NT stores are weakly
ordered on x86, and requires a sfence to be ordered with non-NT stores.
Unfortunately, this per-memcpy sfence instruction make even 1500-byte
sized copy operations much slower - at least in micro benchmarks.
I agree that calling rte_mb() from each NT memcpy would be counterproductive on
small/medium copy operations.
There's no need for a full barrier after the copy operation. sfence
(e.g., rte_wmb()) is enough. I guess you will want a rte_rmb() before
you start copying as well, if you are using NT loads.
Which is why rte_mb() is not called by the NT memcpy function itself, but by
the application. This requirement will be part of the function's documentation.
rte_memcpy_surprise()
:)
The application must call rte_mb() before it accesses the copy.
Alternatively, the application can call rte_mb() after a burst of NT memcopies.
IA SW optimization manual:
9.4.2 Streaming Store Usage Models
The two primary usage domains for streaming store are coherent
requests
and non-coherent requests.
9.4.2.1 Coherent Requests
Coherent requests are normal loads and stores to system memory,
which
may also hit cache lines
present in another processor in a multiprocessor environment. With
coherent requests, a streaming store
can be used in the same way as a regular store that has been mapped
with a WC memory type (PAT or
MTRR). An SFENCE instruction must be used within a producer-consumer
usage model in order to ensure
coherency and visibility of data between processors.
Within a single-processor system, the CPU can also re-read the same
memory location and be assured of
coherence (that is, a single, consistent view of this memory
location).
The same is true for a multiprocessor
(MP) system, assuming an accepted MP software producer-consumer
synchronization policy is
employed.
With this reference, I am convinced that you are right about the
SFENCE. This puts a checkmark on this item on my TODO list for the
patch. Thank you, Konstantin!
Any ARM CPU experts on the mailing list seeing this, not on vacation?
@Honnappa, I'm looking at you. :-)
Summing up, the question is:
After a bunch of *non-temporal* stores (STNP instruction) on ARM
architecture, does calling rte_wmb() suffice to ensure the data is
visible across the system?
