On 04.07.23 10:06, Mattias Nissler wrote:
This series adds basic support for message-based DMA in qemu's vfio-user
server. This is useful for cases where the client does not provide file
descriptors for accessing system memory via memory mappings. My motivating use
case is to hook up device models as PCIe endpoints to a hardware design. This
works by bridging the PCIe transaction layer to vfio-user, and the endpoint
does not access memory directly, but sends memory requests TLPs to the hardware
design in order to perform DMA.
Note that in addition to the 3 commits included, we also need a
subprojects/libvfio-user roll to bring in this bugfix:
https://github.com/nutanix/libvfio-user/commit/bb308a2e8ee9486a4c8b53d8d773f7c8faaeba08
Stefan, can I ask you to kindly update the
https://gitlab.com/qemu-project/libvfio-user mirror? I'll be happy to include
an update to subprojects/libvfio-user.wrap in this series.
Finally, there is some more work required on top of this series to get
message-based DMA to really work well:
* libvfio-user has a long-standing issue where socket communication gets messed
up when messages are sent from both ends at the same time. See
https://github.com/nutanix/libvfio-user/issues/279 for more details. I've
been engaging there and plan to contribute a fix.
* qemu currently breaks down DMA accesses into chunks of size 8 bytes at
maximum, each of which will be handled in a separate vfio-user DMA request
message. This is quite terrible for large DMA acceses, such as when nvme
reads and writes page-sized blocks for example. Thus, I would like to improve
qemu to be able to perform larger accesses, at least for indirect memory
regions. I have something working locally, but since this will likely result
in more involved surgery and discussion, I am leaving this to be addressed in
a separate patch.
I remember asking Stefan in the past if there wouldn't be a way to avoid
that mmap dance (and also handle uffd etc. easier) for vhost-user
(especially, virtiofsd) by only making QEMU access guest memory.
That could make memory-backend-ram support something like vhost-user,
avoiding shared memory and everything that comes with that (e.g., no
KSM, no shared zeropage).
So this series tackles vfio-user, does anybody know what it would take
to get something similar running for vhost-user?
--
Cheers,
David / dhildenb
