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

[Marcel Apfelbaum]

On 12/14/2016 10:10 PM, Laine Stump wrote:
> On 12/14/2016 12:17 PM, Marcel Apfelbaum wrote:
>> 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,
>
> I don't think of it as "one device" but as "only slot 0 can be used" (since, as Marcel pointed out, a different device can be placed on each of the 8 functions of that slot).
>
>> 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.
>

Hi Laine,

> Is that a fixed thing? I mean, is it always possible to use all 8 functions of a slot (assuming no hotplug of course)?
>

Yes, you simply add to the function 0 on command line:  with -device ...,multifunction=on.

Thanks,
Marcel


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