[libvirt-users] [Qemu-devel] VFIO PCIe Extended Capabilities

Tue Jul 19 20:39:26 UTC 2016

Hi Marcel,

>> Indeed, if a device is attached to a PCI bus it makes no sense to advertise the extended configuration space.
>> Can you please share the QEMU command line? Maybe is possible to make the device's bus PCIe in QEMU?

Changing the following should make the tweak I proposed irrelevant.

- device vfio-pci,host=03:00.0,id=hostdev0,bus=pci.2,addr=0x4
+ device vfio-pci,host=03:00.0,id=hostdev0,bus=pcie.0,addr=0x4

Here's the full command for reference.

2016-07-19 18:42:22.110+0000: starting up libvirt version: 1.2.17, package: 13.el7 (CentOS BuildSystem <http://bugs.centos.org>, 2015-11-20-16:24:10, worker1.bsys.centos.org), qemu version: 2.3.0 (qemu-kvm-ev-2.3.0-31.el7_2.10.9)
LC_ALL=C PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin QEMU_AUDIO_DRV=none /usr/libexec/qemu-kvm -name instance-00000019 -S -machine pc-q35-rhel7.2.0,accel=kvm,usb=off -cpu Haswell-noTSX,+abm,+pdpe1gb,+rdrand,+f16c,+osxsave,+dca,+pdcm,+xtpr,+tm2,+est,+smx,+vmx,+ds_cpl,+monitor,+dtes64,+pbe,+tm,+ht,+ss,+acpi,+ds,+vme -m 8192 -realtime mlock=off -smp 8,sockets=8,cores=1,threads=1 -uuid 8b6865eb-2118-430e-a7e0-2989696576b1 -smbios type=1,manufacturer=Fedora Project,product=OpenStack Nova,version=13.1.0-1.el7,serial=a1068a93-24d1-4da2-8903-f9b8307fb0d8,uuid=8b6865eb-2118-430e-a7e0-2989696576b1,family=Virtual Machine -no-user-config -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/domain-instance-00000019/monitor.sock,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc,driftfix=slew -global kvm-pit.lost_tick_policy=discard -no-hpet -no-shutdown -boot strict=on -device i82801b11-bridge,id=pci.1,bus=pcie.0,addr=0x1e -device pci-bridge,chassis_nr=2,id=pci.2,bus=pci.1,addr=0x1 -drive file=/var/lib/nova/instances/8b6865eb-2118-430e-a7e0-2989696576b1/disk,if=none,id=drive-virtio-disk0,format=qcow2,cache=none -device virtio-blk-pci,scsi=off,bus=pci.2,addr=0x2,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -drive file=/var/lib/nova/instances/8b6865eb-2118-430e-a7e0-2989696576b1/disk.swap,if=none,id=drive-virtio-disk1,format=qcow2,cache=none -device virtio-blk-pci,scsi=off,bus=pci.2,addr=0x3,drive=drive-virtio-disk1,id=virtio-disk1 -netdev tap,fd=29,id=hostnet0,vhost=on,vhostfd=30 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=fa:16:3e:9c:cf:60,bus=pci.2,addr=0x1 -chardev file,id=charserial0,path=/var/lib/nova/instances/8b6865eb-2118-430e-a7e0-2989696576b1/console.log -device isa-serial,chardev=charserial0,id=serial0 -chardev pty,id=charserial1 -device isa-serial,chardev=charserial1,id=serial1 -vnc 0.0.0.0:0 -k en-us -device cirrus-vga,id=video0,bus=pcie.0,addr=0x1 -device vfio-pci,host=03:00.0,id=hostdev0,bus=pci.2,addr=0x4 -device virtio-balloon-pci,id=balloon0,bus=pci.2,addr=0x5 -msg timestamp=on
char device redirected to /dev/pts/3 (label charserial1)

It seems very reasonable to change the qemu command line.  However, I'm using OpenStack Nova to launch the instance, which then uses libvirt, which finally uses qemu.  So, the issues is very buried.

> I think that any instance of a q35 machine where the assigned device is
> placed on the conventional PCI bridge will create this scenario.  It's
> the default for attaching devices to a libvirt managed q35 VM AFAIK.

> Yes, I discussed with Laine from libvirt the possibility to assign
> devices to a PCIe port instead.

Yes, I see this as the issue as well. The VM is being created by libvirt with the following xml.  The problem is that the device is auto-assigned to the pci-bridge by default.

<domain type="kvm">
  <uuid>8b6865eb-2118-430e-a7e0-2989696576b1</uuid>
  <name>instance-00000019</name>
  <memory>8388608</memory>
  <vcpu>8</vcpu>
    ... <snip> ...
  <devices>
    ... <snip> ...
    <hostdev mode="subsystem" type="pci" managed="yes">
      <source>
        <address bus="0x03" domain="0x0000" function="0x0" slot="0x00"/>
      </source>
    </hostdev>
    ... <snip> ...
  </devices>
</domain>

If I do a dumpxml I get the following

<domain type="kvm">
  <uuid>8b6865eb-2118-430e-a7e0-2989696576b1</uuid>
  <name>instance-00000019</name>
  <memory>8388608</memory>
  <vcpu>8</vcpu>
    ... <snip> ...
  <devices>
    ... <snip> ...
    <controller type='pci' index='0' model='pcie-root'>
      <alias name='pcie.0'/>
    </controller>
    <controller type='pci' index='1' model='dmi-to-pci-bridge'>
      <alias name='pci.1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x1e' function='0x0'/>
    </controller>
    <controller type='pci' index='2' model='pci-bridge'>
      <alias name='pci.2'/>
      <address type='pci' domain='0x0000' bus='0x01' slot='0x01' function='0x0'/>
    </controller>
    <interface type='bridge'>
    ... <snip> ...
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <driver name='vfio'/>
      <source>
        <address domain='0x0000' bus='0x03' slot='0x00' function='0x0'/>
      </source>
      <alias name='hostdev0'/>
      <address type='pci' domain='0x0000' bus='0x02' slot='0x02' function='0x0'/>
    </hostdev>
  </devices>
</domain>

If I manually change the domain as follows

    <hostdev mode='subsystem' type='pci' managed='yes'>
      <driver name='vfio'/>
      <source>
        <address domain='0x0000' bus='0x03' slot='0x00' function='0x0'/>
      </source>
      <alias name='hostdev0'/>
-     <address type='pci' domain='0x0000' bus='0x02' slot='0x02' function='0x0'/>
+    <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </hostdev>

I can reimport and the device and it attaches successfully to the pcie-root hub.  The problem is I need to either manually specify this in nova or update the behavior of libvirt to auto assign pcie devices to pcie busses.  This would involve reading pci configuration space to check if the device is a pcie devices.  Then attaching to the pcie-root, if possible.

>>>
>>> In fact, I've tried to fix this multiple times:
>>>
>>> https://lists.nongnu.org/archive/html/qemu-devel/2015-10/msg05384.html
>>> https://lists.nongnu.org/archive/html/qemu-devel/2015-11/msg02422.html
>>> https://lists.nongnu.org/archive/html/qemu-devel/2016-01/msg03259.html
>>>
>>> Yet the patch remains unapplied :(
>>
>> I thought is it in already. Maybe Michael can add it as part of the hard freeze.
>> And if the patch will be applied, the tweak above wouldn't help, right Alex?
>
> The tweak Spenser suggested above should be unnecessary with my proposed
> patch applied.

> I think I finally understand this. The bus is not pcie -> we return
> from vfio_add_ext_cap without "breaking" the extended capabilities
> chain and the bare metal SR-IOV capability will be visible to guest.
> With your patch the PCI bridge will "interfere" and mask the extended
> configuration space completely.

>   Only now searching for that patch did I notice Michael's
> comment hidden at the bottom of his reply, which I assume is why it
> never got applied:
>
> https://patchwork.kernel.org/patch/8057411/
>

> I just saw it too! It seems Michael wants to cache this info
> in device instead of re-calculating it every time.

>> Anyway, the current behavior is clearly a bug, so QEMU hard freeze
>> should be irrelevant.  If anyone wants to take over the patch, feel
>> free.  Thanks,

> I suppose I can handle it, but sadly not for 2.7.
> If Spencer has some time now he can help by testing it and reviewing it quickly :)

I'd be happy to help; but that patch really just more permanently breaks my current workaround ;-) .

Thanks,
Spenser

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