[libvirt] [PATCH v2 RESEND 00/12] PCI passthrough support on s390

[Andrea Bolognani]

On Wed, 2018-07-25 at 16:34 +0800, Yi Min Zhao wrote:
> 在 2018/7/24 下午11:43, Andrea Bolognani 写道:
> > > > More concrete questions: one of the zPCI test cases includes
> > > > 
> > > >       <controller type='pci' index='1' model='pci-bridge'/>
> > > >       <hostdev mode='subsystem' type='pci' managed='no'>
> > > >         <driver name='vfio'/>
> > > >         <source>
> > > >           <address domain='0xffff' bus='0x00' slot='0x00' function='0x0'/>
> > > >         </source>
> > > >         <address type='pci' domain='0x0000' bus='0x01' slot='0x1f'
> > > >                  function='0x0' uid='0xffff' fid='0xffffffff'/>
> > > >       </hostdev>
> > > > 
> > > > which translates to
> > > > 
> > > >     -device zpci,uid=3,fid=2,target=pci.1,id=zpci3 \
> > > >     -device pci-bridge,chassis_nr=1,id=pci.1,bus=pci.0,addr=0x1 \
> > > >     -device zpci,uid=65535,fid=4294967295,target=hostdev0,id=zpci65535 \
> > > >     -device vfio-pci,host=ffff:00:00.0,id=hostdev0,bus=pci.1,addr=0x1f \
> > > > 
> > > > How does the pci-bridge controller show up in the guest, if at all?
> 
> Qemu hides pci-bridge devices and just exposes pci devices to the guest.
> In above example, indeed, qemu will generate a pci-bridge device and it will
> be existing in pci topology. But the guest can't see it. This is very
> special.

Yeah, that's kinda problematic as it violates the principle of least
surprise... If s390 guests can only see a flat PCI topology, then we
should find a way to reject bridges altogether instead of allowing
the user to create them (or even create them automatically) only for
them not to show up in the guest.

> > > > Do the bus= and addr= attributes of vfio-pci and pci-bridge in the
> > > > example above matter eg. for migration purposes?
> 
> Do you mean we leave attribute generation for bus and addr to qemu?

That would be the idea, but of course it can only work if the
address of the underlying PCI device can change without affecting
the guest in any way, including migration. If that's not the case,
and the PCI address needs to be as stable as the IDs, then I don't
think there's a way to avoid storing it in the guest XML, no matter
how confusing that will end up looking.

-- 
Andrea Bolognani / Red Hat / Virtualization