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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
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3
hw/core/machine.c
View file

@ -29,6 +29,9 @@
GlobalProperty hw_compat_4_2[] = {
{ "virtio-blk-device", "x-enable-wce-if-config-wce", "off" },
{ "virtio-blk-device", "seg-max-adjust", "off"},
{ "virtio-scsi-device", "seg_max_adjust", "off"},
{ "vhost-blk-device", "seg_max_adjust", "off"},
};
const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);