[libvirt] [Qemu-devel] [RFC] qmp: query-device-slots command

[Eduardo Habkost]

On Tue, Dec 13, 2016 at 04:15:18PM +0200, Marcel Apfelbaum wrote:
> On 12/13/2016 02:42 PM, Eduardo Habkost wrote:
> > On Tue, Dec 13, 2016 at 12:04:17PM +0100, Markus Armbruster wrote:
> > > Quick interface review only:
> > > 
> > > Eduardo Habkost <ehabkost at redhat.com> writes:
> > > 
> > > > This adds a new command to QMP: query-device-slots. It will allow
> > > > management software to query possible slots where devices can be
> > > > plugged.
> > > > 
> > > > This implementation of the command will return:
> > > > 
> > > > * Multiple PCI slots per bus, in the case of PCI buses;
> > > > * One slot per bus in the case of the other buses;
> > > 
> > > Umm, that doesn't seem right.  For instance, a SCSI bus has multiple
> > > slots.  The slot address is the SCSI ID.  An IDE bus may have one (SATA)
> > > or two (PATA).  For more examples, see qdev-device-use.txt section
> > > "Specifying Bus and Address on Bus".
> > 
> > Yes, I should have clarified that: this version changes only PCI
> > to expose multiple slots, but other buses also need be changed to
> > implement BusClass::enumerate_slots() properly, too.
> > 
> > > 
> > > > * One slot for each entry from query-hotpluggable-cpus.
> > > > In the example below, I am not sure if the PCIe ports are all
> > > > supposed to report all slots, but I didn't find any existing
> > > > field in PCIBus that would help me figure out the actual number
> > > > of slots in a given PCI bus.
> > > > 
> > > > Git tree
> > > > --------
> > > > 
> > > > This patch needs the previous query-machines series I am working
> > > > on. The full tree can be found on the git tree at:
> > > > 
> > > >   git://github.com/ehabkost/qemu-hacks.git work/query-machines-bus-info
> > > > 
> > > > Example output
> > > > --------------
> > > > 
> > > > The following output was returned by QEMU when running it as:
> > > > 
> > > >  $ qemu-system-x86_64 -machine q35 \
> > > >    -readconfig docs/q35-chipset.cfg \
> > > >    -smp 4,maxcpus=8,sockets=2,cores=2,threads=2
> > > > 
> > > >  {
> > > >     "return": [
> > > 
> > > Are you sure >3000 lines of example output make sense here?
> > 
> > I'm not. :)
> > 
> > That's why I need feedback from the PCI experts. I believe most
> > of the PCI ports on q35 accept only one device, but I see no code
> > implementing that restriction, and no obvious PCIBus or
> > PCIBusClass field indicating that.
> 
> Hi,
> 
> Sadly there is no such restriction today as far as I know.
> We trust the user that he knows what he is doing :)
> and we let libvirt implement the restriction.
> 
> I do agree we should fix that.

OK, this matches what I saw on the current code.

But I would like to understand what exactly should be the rules
if we fix that. Should we report only 1 slot if
pci_bus_has_pcie_upstream_port(bus)?

(Note that pci_bus_has_pcie_upstream_port() doesn't exist in the
current code, it is added by my patch)

-- 
Eduardo