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virtio: make seg_max virtqueue size dependent

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Before the patch, seg_max parameter was immutable and hardcoded
to 126 (128 - 2) without respect to queue size. This has two negative effects:

1. when queue size is < 128, we have Virtio 1.1 specfication violation:
   (2.6.5.3.1 Driver Requirements) seq_max must be <= queue_size.
   This violation affects the old Linux guests (ver < 4.14). These guests
   crash on these queue_size setups.

2. when queue_size > 128, as was pointed out by Denis Lunev <den@virtuozzo.com>,
   seg_max restrics guest's block request length which affects guests'
   performance making them issues more block request than needed.
   https://lists.gnu.org/archive/html/qemu-devel/2017-12/msg03721.html

To mitigate this two effects, the patch adds the property adjusting seg_max
to queue size automaticaly. Since seg_max is a guest visible parameter,
the property is machine type managable and allows to choose between
old (seg_max = 126 always) and new (seg_max = queue_size - 2) behaviors.

Not to change the behavior of the older VMs, prevent setting the default
seg_max_adjust value for older machine types.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
Message-Id: <20191220140905.1718-2-dplotnikov@virtuozzo.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
This commit is contained in:
Denis Plotnikov 2019-12-20 17:09:04 +03:00 • committed by Michael S. Tsirkin
parent d940208cbb
commit 1bf8a989a5
6 changed files with 24 additions and 2 deletions
Download patch file Download diff file Expand all files Collapse all files

2
hw/scsi/vhost-scsi.c
View file

@ -275,6 +275,8 @@ static Property vhost_scsi_properties[] = {
DEFINE_PROP_UINT32("num_queues", VirtIOSCSICommon, conf.num_queues, 1),
DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSICommon, conf.virtqueue_size,
128),
DEFINE_PROP_BOOL("seg_max_adjust", VirtIOSCSICommon, conf.seg_max_adjust,
true),
DEFINE_PROP_UINT32("max_sectors", VirtIOSCSICommon, conf.max_sectors,
0xFFFF),
DEFINE_PROP_UINT32("cmd_per_lun", VirtIOSCSICommon, conf.cmd_per_lun, 128),

10
hw/scsi/virtio-scsi.c
View file

@ -659,7 +659,8 @@ static void virtio_scsi_get_config(VirtIODevice *vdev,
VirtIOSCSICommon *s = VIRTIO_SCSI_COMMON(vdev);
virtio_stl_p(vdev, &scsiconf->num_queues, s->conf.num_queues);
virtio_stl_p(vdev, &scsiconf->seg_max, 128 - 2);
virtio_stl_p(vdev, &scsiconf->seg_max,
s->conf.seg_max_adjust ? s->conf.virtqueue_size - 2 : 128 - 2);
virtio_stl_p(vdev, &scsiconf->max_sectors, s->conf.max_sectors);
virtio_stl_p(vdev, &scsiconf->cmd_per_lun, s->conf.cmd_per_lun);
virtio_stl_p(vdev, &scsiconf->event_info_size, sizeof(VirtIOSCSIEvent));
@ -898,6 +899,11 @@ void virtio_scsi_common_realize(DeviceState *dev,
virtio_cleanup(vdev);
return;
}
if (s->conf.virtqueue_size <= 2) {
error_setg(errp, "invalid virtqueue_size property (= %" PRIu32 "), "
"must be > 2", s->conf.virtqueue_size);
return;
}
s->cmd_vqs = g_new0(VirtQueue *, s->conf.num_queues);
s->sense_size = VIRTIO_SCSI_SENSE_DEFAULT_SIZE;
s->cdb_size = VIRTIO_SCSI_CDB_DEFAULT_SIZE;
@ -954,6 +960,8 @@ static Property virtio_scsi_properties[] = {
DEFINE_PROP_UINT32("num_queues", VirtIOSCSI, parent_obj.conf.num_queues, 1),
DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSI,
parent_obj.conf.virtqueue_size, 128),
DEFINE_PROP_BOOL("seg_max_adjust", VirtIOSCSI,
parent_obj.conf.seg_max_adjust, true),
DEFINE_PROP_UINT32("max_sectors", VirtIOSCSI, parent_obj.conf.max_sectors,
0xFFFF),
DEFINE_PROP_UINT32("cmd_per_lun", VirtIOSCSI, parent_obj.conf.cmd_per_lun,