Re: [Qemu-devel] [Question] Why doesn't PCIe hotplug work for Q35 machine?

[Gonglei (Arei)]

> >> Subject: Re: [Question] Why doesn't PCIe hotplug work for Q35 machine?
> >>
> >> On Sun, 2014-08-17 at 13:00 +0200, Michael S. Tsirkin wrote:
> >>> On Fri, Aug 15, 2014 at 07:33:29AM +0000, Gonglei (Arei) wrote:
> >>>> Hi,
> >>>>
> >>>> I noticed that the qemu-2.1 release change log says
> >>>> " PCIe: Basic hot-plug/hot-unplug support for Q35 machine."
> >>>> And then I made a testing for the hotplugging function of Q35.
> >>>> But I'm failed, and I got the dmesg log in guest os as below:
> >>>>
> >>>> [ 159.035250] Pciehp 0000:05:00.0:pcie24: Button pressed on Slot (0 - 4)
> >>>> [ 159.035274] Pciehp 0000:05:00.0:pcie24: Card present on Slot (0 - 4)
> >>>> [ 159.036517] Pciehp 0000:05:00.0:pcie24: PCI slot #0 - 4 - powering on
> due
> >> to button press.
> >>>> [ 159.188049] Pciehp 0000:05:00.0:pcie24: Failed to check link status
> >>>> [ 159.201968] Pciehp 0000:05:00.0:pcie24: Card not present on Slot (0 - 
> >>>> 4)
> >>>> [ 159.202529] Pciehp 0000:05:00.0:pcie24: Already disabled on Slot (0 - 
> >>>> 4)
> >>>>
> >>>> Steps of testing:
> >>>>
> >>>> #1. QEMU version:
> >>>>
> >>>>  The lateset master tree source.
> >>>>
> >>>> #2. Command line:
> >>>>
> >>>> ./qemu-system-x86_64 -enable-kvm -m 2048 -machine q35 -device
> >> ide-drive,bus=ide.2,drive=MacHDD \
> >>>>  -drive
> id=MacHDD,if=none,file=/mnt/sdb/gonglei/image/redhat_q35.img
> >> -monitor stdio -vnc :10 -readconfig ../docs/q35-chipset.cfg
> >>>> QEMU 2.0.93 monitor - type 'help' for more information
> >>>> (qemu) device_add
> >> virtio-net-pci,id=nic2,bus=pcie-switch-downstream-port-1-1,addr=1.0
> >>>
> >>> I don't think you can use any slot except slot 0 for pci express.
> >
> > OK. Does the PCIe specification say that?
> > I appreciate very much that you explain more.
> 
> The closest I could find is in "7.3. Configuration Transaction
> Rules"/"7.3.1. Device Number":
> 
> With non-ARI Devices, PCI Express components are restricted to
> implementing a single Device Number on their primary interface (Upstream
> Port) [...] Downstream Ports that do not have ARI Forwarding enabled
> must associate only Device 0 with the device attached to the Logical Bus
> representing the Link from the Port. Configuration Requests
> targeting the Bus Number associated with a Link specifying Device Number
> 0 are delivered to the device attached to the Link; Configuration
> Requests specifying all other Device Numbers (1-31)
> must be terminated by the Switch Downstream Port or the Root Port with
> an Unsupported Request Completion Status (equivalent to Master Abort in
> PCI).
> 
Thanks a lot, Paolo.

And I found another issue when cold-plugging don't using slot 0, the PCIe 
device also can't
be searched in guest os. 

So, I have some questions and ideas:

1. Does qemu support ARI Forwarding for PCIe at present? If yes, how to enable 
it ?
2. If not, we should add some check for PCIe root ports and downstream ports,
 meanwhile add explaining document.
3. Those check should add in general code level, both hotplug and coldplug.

Am I right?  Thanks.

Best regards,
-Gonglei