From 1bf8a989a566b2ba41c197004ec2a02562a766a4 Mon Sep 17 00:00:00 2001 From: Denis Plotnikov Date: Fri, 20 Dec 2019 17:09:04 +0300 Subject: virtio: make seg_max virtqueue size dependent 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 , 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 Signed-off-by: Denis Plotnikov Message-Id: <20191220140905.1718-2-dplotnikov@virtuozzo.com> Reviewed-by: Michael S. Tsirkin Signed-off-by: Michael S. Tsirkin --- hw/core/machine.c | 3 +++ 1 file changed, 3 insertions(+) (limited to 'hw/core/machine.c') diff --git a/hw/core/machine.c b/hw/core/machine.c index f5e2b32b3b..ec2e3fcb61 100644 --- a/hw/core/machine.c +++ b/hw/core/machine.c @@ -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); -- cgit 1.4.1