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

Marcel Apfelbaum marcel at redhat.com
Wed Dec 14 17:17:12 UTC 2016

On 12/13/2016 06:00 PM, Eduardo Habkost wrote:
> 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)?

We should report 1 slot for a bus exposed by a PCIe Root Port
or PCIe Downstream Port.

Be aware that 1 slot doesn't necessarily refer to only one device,
we can have a multi-function device.


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

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