Re: [Qemu-devel] usb-storage assertions

[Gerd Hoffmann]

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?

thanks,
  Gerd