On 2012-07-24 09:42, Nicholas A. Bellinger wrote:
> 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
> qemu.git/master:
>
> 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 <jan.kiszka@xxxxxxxxxxx>
> 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?
Also, is there a git tree and a way to reproduce this without special
hardware needs?
>
> 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.
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?
Jan
--
Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux
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