[PATCH v5 1/2] virtio: make seg_max virtqueue size dependent

[Denis Plotnikov]

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 <address@hidden>,
   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 <address@hidden>
Signed-off-by: Denis Plotnikov <address@hidden>
---
 hw/block/virtio-blk.c           |  9 ++++++++-
 hw/core/machine.c               |  3 +++
 hw/scsi/vhost-scsi.c            |  2 ++
 hw/scsi/virtio-scsi.c           | 10 +++++++++-
 include/hw/virtio/virtio-blk.h  |  1 +
 include/hw/virtio/virtio-scsi.h |  1 +
 6 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index d62e6377c2..0f6f8113b7 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -908,7 +908,8 @@ static void virtio_blk_update_config(VirtIODevice *vdev, 
uint8_t *config)
     blk_get_geometry(s->blk, &capacity);
     memset(&blkcfg, 0, sizeof(blkcfg));
     virtio_stq_p(vdev, &blkcfg.capacity, capacity);
-    virtio_stl_p(vdev, &blkcfg.seg_max, 128 - 2);
+    virtio_stl_p(vdev, &blkcfg.seg_max,
+                 s->conf.seg_max_adjust ? s->conf.queue_size - 2 : 128 - 2);
     virtio_stw_p(vdev, &blkcfg.geometry.cylinders, conf->cyls);
     virtio_stl_p(vdev, &blkcfg.blk_size, blk_size);
     virtio_stw_p(vdev, &blkcfg.min_io_size, conf->min_io_size / blk_size);
@@ -1133,6 +1134,11 @@ static void virtio_blk_device_realize(DeviceState *dev, 
Error **errp)
         error_setg(errp, "num-queues property must be larger than 0");
         return;
     }
+    if (conf->queue_size <= 2) {
+        error_setg(errp, "invalid queue-size property (%" PRIu16 "), "
+                   "must be > 2", conf->queue_size);
+        return;
+    }
     if (!is_power_of_2(conf->queue_size) ||
         conf->queue_size > VIRTQUEUE_MAX_SIZE) {
         error_setg(errp, "invalid queue-size property (%" PRIu16 "), "
@@ -1262,6 +1268,7 @@ static Property virtio_blk_properties[] = {
                     true),
     DEFINE_PROP_UINT16("num-queues", VirtIOBlock, conf.num_queues, 1),
     DEFINE_PROP_UINT16("queue-size", VirtIOBlock, conf.queue_size, 128),
+    DEFINE_PROP_BOOL("seg-max-adjust", VirtIOBlock, conf.seg_max_adjust, true),
     DEFINE_PROP_LINK("iothread", VirtIOBlock, conf.iothread, TYPE_IOTHREAD,
                      IOThread *),
     DEFINE_PROP_BIT64("discard", VirtIOBlock, host_features,
diff --git a/hw/core/machine.c b/hw/core/machine.c
index 023548b4f3..bfa320387e 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);
 
diff --git a/hw/scsi/vhost-scsi.c b/hw/scsi/vhost-scsi.c
index c693fc748a..26f710d3ec 100644
--- a/hw/scsi/vhost-scsi.c
+++ b/hw/scsi/vhost-scsi.c
@@ -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),
diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index e8b2b64d09..405cb6c953 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -654,7 +654,8 @@ 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));
@@ -893,6 +894,11 @@ void virtio_scsi_common_realize(DeviceState *dev,
         virtio_cleanup(vdev);
         return;
     }
+    if (s->conf.virtqueue_size <= 2) {
+        error_setg(errp, "invalid virtqueue_size property (= %" PRIu16 "), "
+                   "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;
@@ -949,6 +955,8 @@ 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,
diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
index 9c19f5b634..1e62f869b2 100644
--- a/include/hw/virtio/virtio-blk.h
+++ b/include/hw/virtio/virtio-blk.h
@@ -38,6 +38,7 @@ struct VirtIOBlkConf
     uint32_t request_merging;
     uint16_t num_queues;
     uint16_t queue_size;
+    bool seg_max_adjust;
     uint32_t max_discard_sectors;
     uint32_t max_write_zeroes_sectors;
     bool x_enable_wce_if_config_wce;
diff --git a/include/hw/virtio/virtio-scsi.h b/include/hw/virtio/virtio-scsi.h
index 122f7c4b6f..24e768909d 100644
--- a/include/hw/virtio/virtio-scsi.h
+++ b/include/hw/virtio/virtio-scsi.h
@@ -48,6 +48,7 @@ typedef struct virtio_scsi_config VirtIOSCSIConfig;
 struct VirtIOSCSIConf {
     uint32_t num_queues;
     uint32_t virtqueue_size;
+    bool seg_max_adjust;
     uint32_t max_sectors;
     uint32_t cmd_per_lun;
 #ifdef CONFIG_VHOST_SCSI
-- 
2.17.0