[PATCH v3 0/7] net/9p: remove msize limit in virtio transport

This series aims to get get rid of the current 500k 'msize' limitation in
the 9p virtio transport, which is currently a bottleneck for performance
of 9p mounts.

This is a follow-up of the following series and discussion:
https://lore.kernel.org/all/28bb651ae0349a7d57e8ddc92c1bd5e62924a912.1630770829.git.linux_oss@crudebyte.com/T/#eb647d0c013616cee3eb8ba9d87da7d8b1f476f37 (local)

Known limitation: With this series applied I can run

  QEMU host <-> 9P virtio <-> Linux guest

with up to 3 MB msize. If I try to run it with 4 MB it seems to hit some
limitation on QEMU side:

  qemu-system-x86_64: virtio: too many write descriptors in indirect table

I haven't looked into this issue yet.

Prerequisites: If you are testing with QEMU then please apply the following
patch on QEMU side:
https://lore.kernel.org/qemu-devel/E1mT2Js-0000DW-OH@lizzy.crudebyte.com/ (local)
That QEMU patch is required if you are using a user space app that
automatically retrieves an optimum I/O block size by obeying stat's
st_blksize, which 'cat' for instance is doing, e.g.:

	time cat test_rnd.dat > /dev/null

Otherwise please use a user space app for performance testing that allows
you to force a large block size and to avoid that QEMU issue, like 'dd'
for instance, in that case you don't need to patch QEMU.

Testing and feedback appreciated!

v2 -> v3:

  * Make vq_sg_free() safe for NULL argument [patch 4].

  * Show info message to user if user's msize option had to be limited in
    case it would exceed p9_max_pages [patch 6].

  * Fix memory leak in vq_sg_resize() [patch 7].

  * Show info message to user if user's msize option had to be limited in
    case not all required SG lists could be allocated [patch 7].

Christian Schoenebeck (7):
  net/9p: show error message if user 'msize' cannot be satisfied
  9p/trans_virtio: separate allocation of scatter gather list
  9p/trans_virtio: turn amount of sg lists into runtime info
  9p/trans_virtio: introduce struct virtqueue_sg
  net/9p: add trans_maxsize to struct p9_client
  9p/trans_virtio: support larger msize values
  9p/trans_virtio: resize sg lists to whatever is possible

 include/net/9p/client.h |   2 +
 net/9p/client.c         |  17 ++-
 net/9p/trans_virtio.c   | 304 +++++++++++++++++++++++++++++++++++-----
 3 files changed, 283 insertions(+), 40 deletions(-)

-- 
2.20.1