Re: [Qemu-devel] usb-storage assertions

[Kevin Wolf]

Am 12.01.2016 um 15:17 hat Gerd Hoffmann geschrieben:
> On Sa, 2016-01-09 at 20:34 +0300, Andrey Korolyov wrote:
> > Hello,
> > 
> > during regular operations within linux guest with USB EHCI frontend I
> > am seeing process crashes with an assert during regular operations
> > like dpkg install:
> > 
> > hw/usb/dev-storage.c:334: usb_msd_handle_reset: Assertion `s->req ==
> > ((void *)0)' failed.
> > 
> > This does happen when real block backend is an USB-attached device
> > itself, so we are hitting some subtle race right there, because a
> > single change of a backend to a raw file sitting on SSD has never
> > triggered this block with the linux guest (but xBSD pre-reboot flush
> > would trigger assertion from time to time in the usb_cancel_packet()
> > without regarding physical backend type:
> > 
> > hw/usb/core.c:508: usb_cancel_packet: Assertion
> > `usb_packet_is_inflight(p)' failed.
> > )
> > 
> > Since the 2.2.0 which I am running is not exactly freshest one, I
> > could re-check with master today or tomorrow, but log for hw/usb/ has
> > no related commits for intentional fixes over one or another issue.
> 
> Checking would be nice, but I likewise don't expect things being
> different on 2.5.
> 
> Most likely these are bugs in rarely taken code paths, probably because
> the guest cancels requests and/or resets device due to timeouts (which
> in turn are triggered by slow backing storage on the host).
> 
> Any chance you can rebuild qemu with DEBUG_MSD enabled (in
> hw/usb/dev-storage.c), then trigger the asserts again, so we hopefully
> get a useful trail of the events triggering the bugs?
> 
> Kevin, is is possible to limit transfer rates for block devices, to
> simplify debugging stuff like this?  Preferably in a way supported by
> libvirt?

Yes. In the qemu command line, you can limit the rate with -drive
throttling.bps_total=... and similar options (you can instead throttle
based on iops instead, and you can have separate read/write limits - I
don't think you need any of these, though).

I have never used it in libvirt, but libvirt.org seems to suggest that
it looks like this (seems to be for Xen, but you get the idea):

    <disk type='file' snapshot='external'>
      <driver name="tap" type="aio" cache="default"/>
      <source file='/var/lib/xen/images/fv0' startupPolicy='optional'>
        <seclabel relabel='no'/>
      </source>
      <target dev='hda' bus='ide'/>
      <iotune>
        <total_bytes_sec>10000000</total_bytes_sec>
        <read_iops_sec>400000</read_iops_sec>
        <write_iops_sec>100000</write_iops_sec>
      </iotune>
    </disk>

Kevin