On Tue, Jul 24, 2012 at 01:20:56PM -0700, Nicholas A. Bellinger wrote:

On Tue, 2012-07-24 at 09:57 +0200, Jan Kiszka wrote:

quoted

On 2012-07-24 09:42, Nicholas A. Bellinger wrote:

quoted

Hi Anthony, Stefan & QEMU folks,

<SNIP>

quoted

quoted

However, thus far I've not been able to get virtio-scsi <-> tcm_vhost
I/O to actually work against the latest qemu.git/master..  

So while doing a (manual) bisection w/ this series to track down the
issue with qemu/master, I managed to run across something else..  With
the vhost-scsi series applied, everything is working as expected up
until the following commit:

commit 1523ed9e1d46b0b54540049d491475ccac7e6421
Author: Jan Kiszka [off-list ref]
Date:   Thu May 17 10:32:39 2012 -0300

    virtio/vhost: Add support for KVM in-kernel MSI injection


This commit ends up triggering the following assert immediately after
starting qemu with virtio-scsi <-> tcm_vhost:

qemu-system-x86_64: /usr/src/qemu.git/hw/msix.c:515:
       msix_unset_vector_notifiers: Assertion `dev->msix_vector_use_notifier &&
                                    dev->msix_vector_release_notifier' failed.

OK, so adding the following hack allows me to boot:

diff --git a/hw/msix.c b/hw/msix.c
index 59c7a83..6036909 100644
--- a/hw/msix.c
+++ b/hw/msix.c

@@ -511,6 +511,11 @@ void msix_unset_vector_notifiers(PCIDevice *dev)
 {
     int vector;
 
+    if (!dev->msix_vector_use_notifier && !dev->msix_vector_release_notifier) {
+        printf("Hit NULL msix_unset_vector_notifiers for: %s\n", dev->name);
+        return;
+    }
+
     assert(dev->msix_vector_use_notifier &&
            dev->msix_vector_release_notifier);
 
--

Can you post a backtrace from gdb?

Sure, w/o the above patch the backtrace with commit 1523ed9e1d looks
like the following:

(gdb) run
Starting program: /usr/src/qemu.git/x86_64-softmmu/qemu-system-x86_64 -enable-kvm -smp 2 -m 2048 -serial file:/tmp/vhost-serial.txt -hda /usr/src/qemu-vhost.git/debian_squeeze_amd64_standard-old.qcow2 -vhost-scsi id=vhost-scsi0,wwpn=naa.600140579ad21088,tpgt=1 -device virtio-scsi-pci,vhost-scsi=vhost-scsi0,event_idx=off
[Thread debugging using libthread_db enabled]
wwpn = "vhost-scsi0" tpgt = "1"
[New Thread 0x7ffff45f8700 (LWP 26508)]
[New Thread 0x7ffff3bf6700 (LWP 26509)]
[New Thread 0x7ffff33f5700 (LWP 26510)]
vhost_scsi_stop
Failed to clear endpoint
qemu-system-x86_64: /usr/src/qemu.git/hw/msix.c:515: msix_unset_vector_notifiers: Assertion `dev->msix_vector_use_notifier && dev->msix_vector_release_notifier' failed.

Program received signal SIGABRT, Aborted.
0x00007ffff5e8b165 in raise () from /lib/libc.so.6
(gdb) bt
#0  0x00007ffff5e8b165 in raise () from /lib/libc.so.6
#1  0x00007ffff5e8df70 in abort () from /lib/libc.so.6
#2  0x00007ffff5e842b1 in __assert_fail () from /lib/libc.so.6
#3  0x00000000004a84a1 in msix_unset_vector_notifiers (dev=0x1463a70) at /usr/src/qemu.git/hw/msix.c:514
#4  0x00000000004d2865 in virtio_pci_set_guest_notifiers (opaque=0x6788, assign=136)
    at /usr/src/qemu.git/hw/virtio-pci.c:703
#5  0x000000000062955f in vhost_dev_stop (hdev=0x126c8a8, vdev=0x1465220) at /usr/src/qemu.git/hw/vhost.c:954
#6  0x0000000000628989 in vhost_scsi_stop (vs=0x126c890, vdev=0x1465220) at /usr/src/qemu.git/hw/vhost-scsi.c:115
#7  0x000000000062f5c9 in virtio_scsi_set_status (vdev=0x1465220, val=<value optimized out>)
    at /usr/src/qemu.git/hw/virtio-scsi.c:631
#8  0x0000000000632082 in virtio_set_status (vdev=0x1465220, val=136 '\210') at /usr/src/qemu.git/hw/virtio.c:507
#9  0x0000000000633410 in virtio_reset (opaque=0x6788) at /usr/src/qemu.git/hw/virtio.c:517

This is strange, code shows virtio_reset calls virtio_set_status
with value 0. Why is it 136 here?
Stack corruption?

We actually did have a memory corruptor with virtio scsi:

You sent patch
virtio-scsi: Fix incorrect VirtIOSCSI->cmd_vqs[0] definition

maybe the bug goes away if you apply that?

#10 0x00000000004d30a9 in virtio_pci_reset (d=0x1463a70) at /usr/src/qemu.git/hw/virtio-pci.c:280
#11 0x00000000004fc909 in qdev_reset_one (dev=0x6788, opaque=0x6788) at /usr/src/qemu.git/hw/qdev.c:207
#12 0x00000000004fc670 in qdev_walk_children (dev=0x1463a70, devfn=0x4fc8f0 <qdev_reset_one>, 
    busfn=0x4fc510 <qbus_reset_one>, opaque=0x0) at /usr/src/qemu.git/hw/qdev.c:372
#13 0x00000000004ae43d in pci_device_reset (dev=0x6788) at /usr/src/qemu.git/hw/pci.c:163
#14 0x00000000004ae64f in pci_bus_reset (bus=0x1415bd0) at /usr/src/qemu.git/hw/pci.c:206
#15 0x00000000004ae699 in pcibus_reset (qbus=0x6788) at /usr/src/qemu.git/hw/pci.c:213
#16 0x00000000004fc710 in qbus_walk_children (bus=0x1415bd0, devfn=0x4fc8f0 <qdev_reset_one>, busfn=0x6, opaque=0x0)
    at /usr/src/qemu.git/hw/qdev.c:349
#17 0x00000000004fc6a3 in qdev_walk_children (dev=<value optimized out>, devfn=0x4fc8f0 <qdev_reset_one>, 
    busfn=0x4fc510 <qbus_reset_one>, opaque=0x0) at /usr/src/qemu.git/hw/qdev.c:379
#18 0x00000000004fc745 in qbus_walk_children (bus=<value optimized out>, devfn=0x4fc8f0 <qdev_reset_one>, 
    busfn=0x4fc510 <qbus_reset_one>, opaque=0x0) at /usr/src/qemu.git/hw/qdev.c:356
#19 0x00000000004d5822 in qemu_system_reset (report=false) at /usr/src/qemu.git/vl.c:1412
#20 0x00000000004d70bb in main (argc=<value optimized out>, argv=<value optimized out>, envp=<value optimized out>)
    at /usr/src/qemu.git/vl.c:3647
(gdb) 

quoted

Also, is there a git tree and a way to reproduce this without special
hardware needs?

I'll push this series + branches to demonstrate the issue into an public
tree this afternoon.

Also, the particular backend is a Fusion-IO raw block flash device, but
I'm pretty sure that using a TCM RAMDISK into tcm_vhost would exhibit
the same type of behavior.  (Will double check on that shortly..)

quoted

quoted

and virtio-scsi is then able to load + detect tcm_vhost LUNs as
expected. 

However the random I/O performance with commit 1523ed9e1d46b is off by a
couple of orders of magnitude, ~6K IOPs compared to ~60K IOPs on raw
block flash using just the previous commit bdd00bdc64ba in Jan's series.

So AFAICT there appears to be a serious performance regression that is
easily reproducible with that patch, which is about as far along as I've
been able to diagnose yet.

Interestingly enough, virtio-scsi-raw performance does not seem to be
effected AFAICT by this regression, and is still able to go ~20K IOPs
with the same workload using commit 1523ed9e1d46b.  (Roughly the same as
before)

Does anyone have any idea why commit 1523ed9e1d46b would be killing
vhost / tcm_vhost performance so terribly, or is there something else
that vhost / vhost-scsi should be doing with new code..?

No good idea yet, will have to look closer.

<nod>, thanks for your help Jan.  ;)

quoted

Maybe you are somehow deassigning (via set_guest_notifiers) before
assigning. But that would not yet explain performance regressions. Your
target is exposing MSI-X, isn't it?

I believe that is correct.

--nab