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>
1bf8a989a5