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

在 2018/7/26 下午7:00, Andrea Bolognani 写道: > > > > > > > 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. > > If we reject bridges, there would be only one pci bus that maximum > > 32 pci devices could be plugged to. This kind of limitation is more > > problematic IMO. > I see how that would be pretty limiting. > > >From the test cases I see a zpci devices, with its own uid and fid, > is created for the pci-bridge as well... Is that intentional? > Firstly pci bridge can be auto-generated if a pci device is to be plugged to non-existing pci bus. IIUC, pci-bridge is treated as a controller device in libvirt. So I think, it's pretty readable not only in libvirt xml but also in qtree, if we assign zpci device for it. Otherwise address type of pci-bridge is pci type but has no uid and fid. Isn't it odd?

From the libvirt side we must avoid any scenario where QEMU auto-adds