vhost-scsi: Reduce response iov mem use

We have to save N iov entries to copy the virtio_scsi_cmd_resp struct
back to the guest's buffer. The difficulty is that we can't assume the
virtio_scsi_cmd_resp will be in 1 iov because older virtio specs allowed
you to break it up.

The worst case is that the guest was doing something like breaking up
the virtio_scsi_cmd_resp struct into 108 (the struct is 108 bytes)
byte sized vecs like:

iov[0].iov_base = ((unsigned char *)virtio_scsi_cmd_resp)[0]
iov[0].iov_len = 1
iov[1].iov_base =  ((unsigned char *)virtio_scsi_cmd_resp)[1]
iov[1].iov_len = 1
....
iov[107].iov_base = ((unsigned char *)virtio_scsi_cmd_resp)[107]
iov[1].iov_len = 1

Right now we allocate UIO_MAXIOV vecs which is 1024 and so for a small
device with just 1 queue and 128 commands per queue, we are wasting

1.8 MB = (1024 current entries - 108) * 16 bytes per entry * 128 cmds

The most common case is going to be where the initiator puts the entire
virtio_scsi_cmd_resp in the first iov and does not split it. We've
always done it this way for Linux and the windows driver looks like
it's always done the same. It's highly unlikely anyone has ever split
the response and if they did it might just be where they have the
sense in a second iov but that doesn't seem likely as well.

So to optimize for the common implementation, this has us only
pre-allocate the single iovec. If we do hit the split up response case
this has us allocate the needed iovec when needed.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20241203191705.19431-9-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

0 files changed, 0 insertions, 0 deletions