Re: [PATCH v2 0/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k

[Stefan Hajnoczi]

On Mon, Oct 04, 2021 at 09:38:00PM +0200, Christian Schoenebeck wrote:
> At the moment the maximum transfer size with virtio is limited to 4M
> (1024 * PAGE_SIZE). This series raises this limit to its maximum
> theoretical possible transfer size of 128M (32k pages) according to the
> virtio specs:
> 
> https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-240006

Hi Christian,
I took a quick look at the code:

- The Linux 9p driver restricts descriptor chains to 128 elements
  (net/9p/trans_virtio.c:VIRTQUEUE_NUM)

- The QEMU 9pfs code passes iovecs directly to preadv(2) and will fail
  with EINVAL when called with more than IOV_MAX iovecs
  (hw/9pfs/9p.c:v9fs_read())

Unless I misunderstood the code, neither side can take advantage of the
new 32k descriptor chain limit?

Thanks,
Stefan

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