[SeaBIOS] Bug in SeaBIOS virtio-ring handling bug with vhost-scsi-pci

Nicholas A. Bellinger nab at linux-iscsi.org
Fri Apr 5 04:59:52 CEST 2013


Hi Paolo & Co,

So I've been running into another bug with SeaBIOS w/ virtio-scsi +
vhost-scsi-pci code..

During the last weeks debugging on the QEMU vhost memory re-mapping bug,
my seabios builds have been using a smaller target ID (16) for scanning
than the default hard coded value of 256 in init_virtio_scsi().

After bumping this value back to 256 recently, a bogus out_num value
from vhost_get_vq_desc() start appearing on the tcm_vhost side:

[17348.044031] Invalid buffer layout out: 0 in: 2

After adding more debug output to SeaBIOS, the vring->num value appears
to be reset from the expected MAX_QUEUE_NUM=128, then incremented each
time after vring_more_used() is called to poll for completion.. 

init virtio-scsi
found virtio-scsi at 0:4
vring_init Setting vr->num: 128
Calling virtio_scsi_scan_target for target: 0
Searching bootorder for: /pci at i0cf8/*@4/*@0/*@0,0
virtio_scsi_cmd, using vring_add_buf for out_num: 1 in_num: 2
virtio_scsi_cmd, using vring_add_buf vr: 0x000ed404 vr->num: 128
vring_add_buf, vr: 0x000ed404 vr->num: 128
vring_add_buf, head: 0, index: 0
vring_add_buf, setting desc[prev=2].flags
vring_add_buf, setting free_head: 3
vring_add_buf, setting q->vdata[head=0]
vring_add_buf, setting q->ring[av=0]
virtio_scsi_cmd, after vring_add_buf vr: 0x000ed404 vr->num: 128
virtio_scsi_cmd, after vring_kick vr: 0x000ed404 vr->num: 128
virtio_scsi_cmd, after vring_more_used #4 vr: 0x000ed404 vr->num: 1
                                          ^^^^^^^^^^^^^^^^^^^^^^^^^   

   Reset from 128 to 1 immediate after first vring_more_used()..?

vring_get_buf last_used_idx: 0 vr: 0x000ed404 vr->num: 1
vring_get_buf checking &used->ring[0]
vring_get_buf, using new last_unsed: 0 >>>>>>>>>>>>>>>>>>
vring_get_buf using id: 0 from elem->id: 0x000ed004
vring_detach: vq->free_head: 3
vring_detach: Setting new vq->free_head: 0
virtio_scsi_cmd, after vring_get_buf #5 vr: 0x000ed404 vr->num: 1
Calling virtio_scsi_scan_target for target: 1
Searching bootorder for: /pci at i0cf8/*@4/*@0/*@1,0
virtio_scsi_cmd, using vring_add_buf for out_num: 1 in_num: 2
virtio_scsi_cmd, using vring_add_buf vr: 0x000ed404 vr->num: 1
vring_add_buf, vr: 0x000ed404 vr->num: 1
vring_add_buf, head: 0, index: 0
vring_add_buf, setting desc[prev=2].flags
vring_add_buf, setting free_head: 3
vring_add_buf, setting q->vdata[head=0]
vring_add_buf, setting q->ring[av=0]
virtio_scsi_cmd, after vring_add_buf vr: 0x000ed404 vr->num: 1
virtio_scsi_cmd, after vring_kick vr: 0x000ed404 vr->num: 1
virtio_scsi_cmd, after vring_more_used #4 vr: 0x000ed404 vr->num: 2
                                          ^^^^^^^^^^^^^^^^^^^^^^^^^

  Incremented from 1 to 2 after second vring_more_used()..?

vring_get_buf last_used_idx: 1 vr: 0x000ed404 vr->num: 2
vring_get_buf checking &used->ring[1]
vring_get_buf, using new last_unsed: 1 >>>>>>>>>>>>>>>>>>
vring_get_buf using id: 0 from elem->id: 0x000ed00c
vring_detach: vq->free_head: 3
vring_detach: Setting new vq->free_head: 0
virtio_scsi_cmd, after vring_get_buf #5 vr: 0x000ed404 vr->num: 2
virtio-scsi vendor='LIO-ORG' product='RAMDISK-MCP' rev='4.0' type=0 removable=0
virtio_scsi_cmd, using vring_add_buf for out_num: 1 in_num: 1
virtio_scsi_cmd, using vring_add_buf vr: 0x000ed404 vr->num: 2
vring_add_buf, vr: 0x000ed404 vr->num: 2
vring_add_buf, head: 0, index: 0
vring_add_buf, setting desc[prev=1].flags
vring_add_buf, setting free_head: 2
vring_add_buf, setting q->vdata[head=0]
vring_add_buf, setting q->ring[av=0]
virtio_scsi_cmd, after vring_add_buf vr: 0x000ed404 vr->num: 2
virtio_scsi_cmd, after vring_kick vr: 0x000ed404 vr->num: 2
virtio_scsi_cmd, after vring_more_used #4 vr: 0x000ed404 vr->num: 3
                                          ^^^^^^^^^^^^^^^^^^^^^^^^^

  Incremented from 2 to 3, and so on..

and this keeps happening until vr->num begins to exceed
MAX_QUEUE_NUM around scanning of target ID 127:

virtio_scsi_cmd, after vring_add_buf vr: 0x000ed404 vr->num: 128
virtio_scsi_cmd, after vring_kick vr: 0x000ed404 vr->num: 128
virtio_scsi_cmd, after vring_more_used #4 vr: 0x000ed404 vr->num: 129
vring_get_buf last_used_idx: 128 vr: 0x000ed404 vr->num: 129
vring_get_buf checking &used->ring[128]
vring_get_buf, using new last_unsed: 128 >>>>>>>>>>>>>>>>>>
vring_get_buf using id: 129 from elem->id: 0x000ed404
vring_detach: vq->free_head: 3
vring_detach: Setting new vq->free_head: 129
virtio_scsi_cmd, after vring_get_buf #5 vr: 0x000ed404 vr->num: 129
Calling virtio_scsi_scan_target for target: 127
Searching bootorder for: /pci at i0cf8/*@4/*@0/*@127,0
virtio_scsi_cmd, using vring_add_buf for out_num: 1 in_num: 2
virtio_scsi_cmd, using vring_add_buf vr: 0x000ed404 vr->num: 129
vring_add_buf, vr: 0x000ed404 vr->num: 129
vring_add_buf, head: 129, index: 0
vring_add_buf, setting desc[prev=4].flags
vring_add_buf, setting free_head: 5
vring_add_buf, setting q->vdata[head=0]
vring_add_buf, setting q->ring[av=129]
virtio_scsi_cmd, after vring_add_buf vr: 0x000ed404 vr->num: 129
virtio_scsi_cmd, after vring_kick vr: 0x000ed404 vr->num: 129
virtio_scsi_cmd, after vring_more_used #4 vr: 0x000ed404 vr->num: 130
vring_get_buf last_used_idx: 129 vr: 0x000ed404 vr->num: 130
vring_get_buf checking &used->ring[129]
vring_get_buf, using new last_unsed: 129 >>>>>>>>>>>>>>>>>>
vring_get_buf using id: 968704 from elem->id: 0x000ed40c
vring_detach: vq->free_head: 5
vring_detach: Setting new vq->free_head: 968704
virtio_scsi_cmd, after vring_get_buf #5 vr: 0x000ed404 vr->num: 130
Calling virtio_scsi_scan_target for target: 128
Searching bootorder for: /pci at i0cf8/*@4/*@0/*@128,0
virtio_scsi_cmd, using vring_add_buf for out_num: 1 in_num: 2
virtio_scsi_cmd, using vring_add_buf vr: 0x000ed404 vr->num: 130
vring_add_buf, vr: 0x000ed404 vr->num: 130
vring_add_buf, head: 51200, index: 0
vring_add_buf, setting desc[prev=1].flags
vring_add_buf, setting free_head: 2
vring_add_buf, setting q->vdata[head=0]
vring_add_buf, setting q->ring[av=51200]

until '[17348.044031] Invalid buffer layout out: 0 in: 2' appears on the
vhost.

So it's clear this definitely is not supposed to be happening, but I've
not yet been able to ascertain why it's suddenly being triggered now..?
I don't recall Asias running into a similar issue during his earlier
testing, but this bug is happening after every I/O in SeaBIOS with the
most recent vhost-scsi-pci code.

I'm using the following to work-around this issue for the moment, and
would like some input from virtio / SeaBIOS folks..

WDYT..?

--nab

diff --git a/src/virtio-ring.c b/src/virtio-ring.c
index bddb090..e046141 100644
--- a/src/virtio-ring.c
+++ b/src/virtio-ring.c
@@ -35,10 +35,16 @@
 
 int vring_more_used(struct vring_virtqueue *vq)
 {
+    struct vring *vr = &vq->vring;
     struct vring_used *used = GET_LOWFLAT(vq->vring.used);
     int more = GET_LOWFLAT(vq->last_used_idx) != GET_LOWFLAT(used->idx);
     /* Make sure ring reads are done after idx read above. */
     smp_rmb();
+
+    /* Huh..?  vr->num is somehow changing from below us..? */
+    if (GET_LOWFLAT(vr->num) != MAX_QUEUE_NUM) {
+           SET_LOWFLAT(vr->num, MAX_QUEUE_NUM);
+    }
     return more;
 }




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