Re: [libvirt] [PATCH v2 1/6] tests: qemuxml2argv: remove some QEMU_CAPS_DEVICE problem cases

> > > > Why did I think that we had eliminated all use of "-usb"? Is it that we're > > > > currently only using it in the tests? If so, that's another thing to eliminate > > > > - we should never be using -usb unless that's the only way to get a usb device > > > > for a particular qemu (and I don't know that that is *ever* the case). > > > >  > > > The qemu_command.c chunk is: > > >  > > >      if (usbcontroller == 0 && > > >          !qemuDomainMachineIsQ35(def) && > > >          !ARCH_IS_S390(def->os.arch)) > > >          virCommandAddArg(cmd, "-usb") > > >  > > > Where usbcontroller == 0 if we didn't build a -device string for a USB > > > controller. I'd need to dig into qemu to figure out what -usb actually maps to > > > to determine if that makes sense anymore > >  > > Okay, that's what I remember. usbcontroller will only be 0 if there was no > > <controller type='usb'/> in the xml, and that shouldn't happen because the > > post-parse will add a usb controller if there isn't any (except in a few cases > > - see qemuDomainDefAddDefaultDevices). So is the post-parse not happening for > > the qemuxml2argvtest? (there are dozens and dozens of "-usb" in the *.args files) >  > PostParse is triggered automatically via DomainDefParse. You can see in the > qemuxml2xml output that there's a USB controller added. >  > Looking deeper, that usbcontroller bit is only set if QEMU_CAPS_PIIX3_USB_UHCI > is available, otherwise it uses legacyusb aka -usb. Maybe that's another flag > that we can assume is available unconditionally for the qemu we support, I'd > need to dig a bit   Just my two cents here.  I remember that we were dealing with this because of ppc64 with Andrea and I think we are using '-usb' in one particular case even when there is the option to specify it as a device.  That case is when we know the default usb controller that used to be added with '-usb' and that particular controller is not available (listed) in the list of devices, so to keep the guest ABI the same, we fallback to '-usb' because we have no other way to do that.  However, this will be a problem in the future when qemu changes the default controller added with -usb.  That's just another rabbit hole I don't want to go too deep in.  Maybe Andrea already knows more and started doing something with it.  Andrea?