在 2024/4/27 星期六 上午 12:14, Gregory Price 写道:
On Fri, Apr 26, 2024 at 10:53:43PM +0800, Dongsheng Yang wrote:
在 2024/4/26 星期五 下午 9:48, Gregory Price 写道:
In (5) of the cover letter, I mentioned that cbd addresses cache coherence
at the software level:
(5) How do blkdev and backend interact through the channel?
a) For reader side, before reading the data, if the data in this channel
may be modified by the other party, then I need to flush the cache before
reading to ensure that I get the latest data. For example, the blkdev needs
to flush the cache before obtaining compr_head because compr_head will be
updated by the backend handler.
b) For writter side, if the written information will be read by others,
then after writing, I need to flush the cache to let the other party see it
immediately. For example, after blkdev submits cbd_se, it needs to update
cmd_head to let the handler have a new cbd_se. Therefore, after updating
cmd_head, I need to flush the cache to let the backend see it.
Flushing the cache is insufficient. All that cache flushing guarantees
is that the memory has left the writer's CPU cache. There are potentially
many write buffers between the CPU and the actual backing media that the
CPU has no visibility of and cannot pierce through to force a full
guaranteed flush back to the media.
for example:
memcpy(some_cacheline, data, 64);
mfence();
Will not guarantee that after mfence() completes that the remote host
will have visibility of the data. mfence() does not guarantee a full
flush back down to the device, it only guarantees it has been pushed out
of the CPU's cache.
similarly:
memcpy(some_cacheline, data, 64);
mfence();
memcpy(some_other_cacheline, data, 64);
mfence()
Will not guarantee that some_cacheline reaches the backing media prior
to some_other_cacheline, as there is no guarantee of write-ordering in
CXL controllers (with the exception of writes to the same cacheline).
So this statement:
I need to flush the cache to let the other party see it immediately.
Is misleading. They will not see is "immediately", they will see it
"eventually at some completely unknowable time in the future".
This is indeed one of the issues I wanted to discuss at the RFC stage.
Thank you for pointing it out.
In my opinion, using "nvdimm_flush" might be one way to address this
issue, but it seems to flush the entire nd_region, which might be too
heavy. Moreover, it only applies to non-volatile memory.
This should be a general problem for cxl shared memory. In theory, FAMFS
should also encounter this issue.
Gregory, John, and Dan, Any suggestion about it?
Thanx a lot
~Gregory
