[PATCH] virtio: fix IO request length in virtio SCSI/block #PSBM-78839

[Denis Plotnikov]

From: "Denis V. Lunev" <address@hidden>

Linux guests submit IO requests no longer than PAGE_SIZE * max_seg
field reported by SCSI controler. Thus typical sequential read with
1 MB size results in the following pattern of the IO from the guest:
  8,16   1    15754     2.766095122  2071  D   R 2095104 + 1008 [dd]
  8,16   1    15755     2.766108785  2071  D   R 2096112 + 1008 [dd]
  8,16   1    15756     2.766113486  2071  D   R 2097120 + 32 [dd]
  8,16   1    15757     2.767668961     0  C   R 2095104 + 1008 [0]
  8,16   1    15758     2.768534315     0  C   R 2096112 + 1008 [0]
  8,16   1    15759     2.768539782     0  C   R 2097120 + 32 [0]
The IO was generated by
  dd if=/dev/sda of=/dev/null bs=1024 iflag=direct

This effectively means that on rotational disks we will observe 3 IOPS
for each 2 MBs processed. This definitely negatively affects both
guest and host IO performance.

The cure is relatively simple - we should report lengthy scatter-gather
ability of the SCSI controller. Fortunately the situation here is very
good. VirtIO transport layer can accomodate 1024 items in one request
while we are using only 128. This situation is present since almost
very beginning. 2 items are dedicated for request metadata thus we
should publish VIRTQUEUE_MAX_SIZE - 2 as max_seg.

The following pattern is observed after the patch:
  8,16   1     9921     2.662721340  2063  D   R 2095104 + 1024 [dd]
  8,16   1     9922     2.662737585  2063  D   R 2096128 + 1024 [dd]
  8,16   1     9923     2.665188167     0  C   R 2095104 + 1024 [0]
  8,16   1     9924     2.665198777     0  C   R 2096128 + 1024 [0]
which is much better.

The dark side of this patch is that we are tweaking guest visible
parameter, though this should be relatively safe as above transport
layer support is present in QEMU/host Linux for a very long time.
The patch adds configurable property for VirtIO SCSI with a new default
and hardcode option for VirtBlock which does not provide good
configurable framework.

Unfortunately the commit can not be applied as is. For the real cure we
need guest to be fixed to accomodate that queue length, which is done
only in the latest 4.14 kernel. Thus we are going to expose the property
and tweak it on machine type level.

The problem with the old kernels is that they have
max_segments <= virtqueue_size restriction which cause the guest
crashing in the case of violation.
To fix the case described above in the old kernels we can increase
virtqueue_size to 256 and max_segments to 254. The pitfall here is
that seabios allows the virtqueue_size-s < 128, however, the seabios
patch extending that value to 256 is pending.

CC: "Michael S. Tsirkin" <address@hidden>
CC: Stefan Hajnoczi <address@hidden>
CC: Kevin Wolf <address@hidden>
CC: Max Reitz <address@hidden>
CC: Gerd Hoffmann <address@hidden>
Signed-off-by: Denis V. Lunev <address@hidden>
Signed-off-by: Denis Plotnikov <address@hidden>
---
 hw/block/virtio-blk.c           | 3 ++-
 hw/scsi/vhost-scsi.c            | 2 ++
 hw/scsi/virtio-scsi.c           | 4 +++-
 include/hw/virtio/virtio-blk.h  | 1 +
 include/hw/virtio/virtio-scsi.h | 1 +
 5 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 06e57a4d39..b2eaeeaf67 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -903,7 +903,7 @@ 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.max_segments);
     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);
@@ -1240,6 +1240,7 @@ static Property virtio_blk_properties[] = {
                        conf.max_discard_sectors, BDRV_REQUEST_MAX_SECTORS),
     DEFINE_PROP_UINT32("max-write-zeroes-sectors", VirtIOBlock,
                        conf.max_write_zeroes_sectors, 
BDRV_REQUEST_MAX_SECTORS),
+    DEFINE_PROP_UINT32("max_segments", VirtIOBlock, conf.max_segments, 126),
     DEFINE_PROP_END_OF_LIST(),
 };
 
diff --git a/hw/scsi/vhost-scsi.c b/hw/scsi/vhost-scsi.c
index 61e2e57da9..fa3b377807 100644
--- a/hw/scsi/vhost-scsi.c
+++ b/hw/scsi/vhost-scsi.c
@@ -242,6 +242,8 @@ static Property vhost_scsi_properties[] = {
     DEFINE_PROP_BIT64("t10_pi", VHostSCSICommon, host_features,
                                                  VIRTIO_SCSI_F_T10_PI,
                                                  false),
+    DEFINE_PROP_UINT32("max_segments", VirtIOSCSICommon, conf.max_segments,
+                       126),
     DEFINE_PROP_END_OF_LIST(),
 };
 
diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 839f120256..8b070ddeed 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -650,7 +650,7 @@ 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.max_segments);
     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));
@@ -948,6 +948,8 @@ static Property virtio_scsi_properties[] = {
                                                 VIRTIO_SCSI_F_CHANGE, true),
     DEFINE_PROP_LINK("iothread", VirtIOSCSI, parent_obj.conf.iothread,
                      TYPE_IOTHREAD, IOThread *),
+    DEFINE_PROP_UINT32("max_segments", VirtIOSCSI, 
parent_obj.conf.max_segments,
+                       126),
     DEFINE_PROP_END_OF_LIST(),
 };
 
diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
index cddcfbebe9..22da23a4a3 100644
--- a/include/hw/virtio/virtio-blk.h
+++ b/include/hw/virtio/virtio-blk.h
@@ -40,6 +40,7 @@ struct VirtIOBlkConf
     uint16_t queue_size;
     uint32_t max_discard_sectors;
     uint32_t max_write_zeroes_sectors;
+    uint32_t max_segments;
 };
 
 struct VirtIOBlockDataPlane;
diff --git a/include/hw/virtio/virtio-scsi.h b/include/hw/virtio/virtio-scsi.h
index 4c0bcdb788..1e5805eec4 100644
--- a/include/hw/virtio/virtio-scsi.h
+++ b/include/hw/virtio/virtio-scsi.h
@@ -49,6 +49,7 @@ struct VirtIOSCSIConf {
     uint32_t num_queues;
     uint32_t virtqueue_size;
     uint32_t max_sectors;
+    uint32_t max_segments;
     uint32_t cmd_per_lun;
 #ifdef CONFIG_VHOST_SCSI
     char *vhostfd;
-- 
2.17.0