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[Qemu-devel] vhost-scsi port to v1.1.0 + MSI-X performance regression

From: Nicholas A. Bellinger
Subject: [Qemu-devel] vhost-scsi port to v1.1.0 + MSI-X performance regression
Date: Tue, 24 Jul 2012 00:42:04 -0700

Hi Anthony, Stefan & QEMU folks,

So during the process of separating out the patches from Zhi's
vhost-scsi tree this evening, I managed to squash everything down to
nine nicely reviewable patches that apply against the current

Nicholas Bellinger (1):
  virtio-scsi: Set max_target=0 during vhost-scsi operation

Stefan Hajnoczi (8):
  notifier: add validity check and notify function
  virtio-pci: support host notifiers in TCG mode
  virtio-pci: check that event notification worked
  vhost: Pass device path to vhost_dev_init()
  virtio-scsi: Add wwpn and tgpt properties
  virtio-scsi: Open and initialize /dev/vhost-scsi
  virtio-scsi: Start/stop vhost
  vhost-scsi: add -vhost-scsi host device

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 <address@hidden>
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 &&

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

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

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..?



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