Re: pci-stub error and MSI-X for KVM guest

[Chris Wright <chrisw@sous-sol.org>]

* Fischer, Anna (anna.fischer@hp.com) wrote:
> > Subject: Re: pci-stub error and MSI-X for KVM guest
> > > This works fine in principle and I can see the PCI device in the
> > > guest under lspci. However, the 82576 VF driver requires the OS
> > > to support MSI-X. My Fedora installation is configured with MSI-X,
> > > e.g. CONFIG_PCI_MSI is 'y'. When I load the driver it tells me it
> > cannot
> > > initialize MSI-X for the device, and under /proc/interrupts I can see
> > > that MSI-X does not seem to work. Is this a KVM/QEMU limitation? It
> > works
> > > for me when running the VF driver under a non-virtualized Linux system.
> > 
> > No, this should work fine.  QEMU/KVM supports MSI-X to guest as well as
> > VFs.
> 
> Actually, I just got this to work. However, it only works if I call
> qemu-kvm from the command line, while it doesn't work when I start
> the guest via the virt-manager. So this seems to be an issue with
> Fedora's virt-manager rather than with KVM/QEMU. If I call qemu-kvm
> from the command line then I get the pci-stub messages saying 'irq xx
> for MSI/MSI-x' when the guest boots up and the VF device works just fine
> inside the guest. When I start the guest using virt-manager then I don't
> see any of these irq allocation messages from pci-stub. Any idea what
> the problem could be here?

No, sounds odd.  Can you:

  # virsh dumpxml [domain]

and show the output of the hostdev XML section?

> > > Also, when I do an lspci on the KVM guest, that is fine, but when I
> > > do an lspci -v then the guest crashes down. In the host OS under dmesg
> > > I can see this:
> > >
> > > pci-stub 0000:03:10.0: restoring config space at offset 0x1 (was
> > 0x100000, writing 0x100004)
> > >
> > > Is this a known issue? My qemu-kvm version is 2:0.11.0.
> > 
> > No, I've not seen the crash before.  What do you mean "the guest crashes
> > down"?
> 
> So this also only happens when starting the guest using virt-manager. It
> works fine when starting qemu-kvm from the command line. This is weird as
> I call it with the same parameters as I can see virt-manager uses under
> 'ps -ef | grep qemu'. The guest crashes down means that the QEMU process
> is terminated. I don't see anything in the logs. It just disappears.

Ouch.  Can you do "debuginfo-install qemu-system-x86" to get the debug
packages, then attach gdb to the QEMU process so that when you do lspci -v
in the guest (assuming this is QEMU segfaulting) you'll get a backtrace?

> > This looks like a Fedora specific version (rpm version).  Can you verify
> > this is from Fedora packages vs. upstream source?  If it's Fedora,
> > would be useful to open a bug there.
> 
> Yes, I am using KVM/QEMU which ships with the Fedora Core 12 distribution.

OK, please file a bug there (and include the backtrace info).

thanks,
-chris