PCI Passthrough and Surprise Hotplug
Alex Williamson
alex.williamson at redhat.com
Wed Oct 7 02:21:29 UTC 2020
On Mon, 5 Oct 2020 11:05:05 -0400
Marc Smith <msmith626 at gmail.com> wrote:
> Hi,
>
> I'm using QEMU/KVM on RHEL (CentOS) 7.8.2003:
> # cat /etc/redhat-release
> CentOS Linux release 7.8.2003
>
> I'm passing an NVMe drive into a Linux KVM virtual machine (<type
> arch='x86_64' machine='pc-i440fx-rhel7.0.0'>hvm</type>) which has the
> following 'hostdev' entry:
> <hostdev mode='subsystem' type='pci' managed='yes'>
> <driver name='vfio'/>
> <source>
> <address domain='0x0000' bus='0x42' slot='0x00' function='0x0'/>
> </source>
> <alias name='hostdev5'/>
> <rom bar='off'/>
> <address type='pci' domain='0x0000' bus='0x01' slot='0x0f'
> function='0x0'/>
> </hostdev>
>
> This all works fine during normal operation, but I noticed when we
> remove the NVMe drive (surprise hotplug event), the PCIe EP then seems
> "stuck"... here we see the link-down event on the host (when the drive
> is removed):
> [67720.177959] pciehp 0000:40:01.2:pcie004: Slot(238-1): Link Down
> [67720.178027] vfio-pci 0000:42:00.0: Relaying device request to user (#0)
>
> And naturally, inside of the Linux VM, we see the NVMe controller drop:
> [ 1203.491536] nvme nvme1: controller is down; will reset:
> CSTS=0xffffffff, PCI_STATUS=0xffff
> [ 1203.522759] blk_update_request: I/O error, dev nvme1n2, sector
> 33554304 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
> [ 1203.560505] nvme 0000:01:0f.0: Refused to change power state, currently in D3
> [ 1203.561104] nvme nvme1: Removing after probe failure status: -19
> [ 1203.583506] Buffer I/O error on dev nvme1n2, logical block 4194288,
> async page read
> [ 1203.583514] blk_update_request: I/O error, dev nvme1n1, sector
> 33554304 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
>
> We see this EP is found at IOMMU group '76':
> # readlink /sys/bus/pci/devices/0000\:42\:00.0/iommu_group
> ../../../../kernel/iommu_groups/76
>
> And it is no longer bound to the 'vfio-pci' driver (expected) on the
> host. I was expecting to see all of the FD's to the /dev/vfio/NN
> character devices closed, but it seems they are still open:
> # lsof | grep "vfio/76"
> qemu-kvm 242364 qemu 70u CHR 235,4
> 0t0 3925324 /dev/vfio/76
> qemu-kvm 242364 242502 qemu 70u CHR 235,4
> 0t0 3925324 /dev/vfio/76
> qemu-kvm 242364 242511 qemu 70u CHR 235,4
> 0t0 3925324 /dev/vfio/76
> qemu-kvm 242364 242518 qemu 70u CHR 235,4
> 0t0 3925324 /dev/vfio/76
> qemu-kvm 242364 242531 qemu 70u CHR 235,4
> 0t0 3925324 /dev/vfio/76
> qemu-kvm 242364 242533 qemu 70u CHR 235,4
> 0t0 3925324 /dev/vfio/76
> qemu-kvm 242364 242542 qemu 70u CHR 235,4
> 0t0 3925324 /dev/vfio/76
> qemu-kvm 242364 242550 qemu 70u CHR 235,4
> 0t0 3925324 /dev/vfio/76
> qemu-kvm 242364 242554 qemu 70u CHR 235,4
> 0t0 3925324 /dev/vfio/76
> SPICE 242364 242559 qemu 70u CHR 235,4
> 0t0 3925324 /dev/vfio/76
>
> After the NVMe drive was removed for 100 seconds, we see the following
> kernel messages on the host:
> [67820.179749] vfio-pci 0000:42:00.0: Relaying device request to user (#10)
> [67900.272468] vfio_bar_restore: 0000:42:00.0 reset recovery - restoring bars
> [67900.272652] vfio_bar_restore: 0000:42:00.0 reset recovery - restoring bars
> [67900.319284] vfio_bar_restore: 0000:42:00.0 reset recovery - restoring bars
>
> I also noticed these messages related to the EP that is down currently
> that seem to continue indefinitely on the host (every 100 seconds):
> [67920.181882] vfio-pci 0000:42:00.0: Relaying device request to user (#20)
> [68020.184945] vfio-pci 0000:42:00.0: Relaying device request to user (#30)
> [68120.188209] vfio-pci 0000:42:00.0: Relaying device request to user (#40)
> [68220.190397] vfio-pci 0000:42:00.0: Relaying device request to user (#50)
> [68320.192575] vfio-pci 0000:42:00.0: Relaying device request to user (#60)
>
> But perhaps that is expected behavior. In any case, the problem comes
> when I re-insert the NVMe drive into the system... on the host, we see
> the link-up event:
> [68418.595101] pciehp 0000:40:01.2:pcie004: Slot(238-1): Link Up
>
> But the device is not bound to the 'vfio-pci' driver:
> # ls -ltr /sys/bus/pci/devices/0000\:42\:00.0/driver
> ls: cannot access /sys/bus/pci/devices/0000:42:00.0/driver: No such
> file or directory
>
> And appears to fail when attempting to bind to it manually:
> # echo "0000:42:00.0" > /sys/bus/pci/drivers/vfio-pci/bind
> -bash: echo: write error: No such device
>
> Device is enabled:
> # cat /sys/bus/pci/devices/0000\:42\:00.0/enable
> 1
>
> So, wondering if this is expected behavior? Stopping the VM and
> starting it (virsh destroy/start) allows the device to work in the VM
> again, but for my particular use case, this is not an option. Need the
> surprise hotplug functionality to work with the PCIe EP passed into
> the VM. And perhaps this is an issue elsewhere (eg, vfio-pci). Any
> tips/suggestions on where to dig more would be appreciated.
Sorry, but nothing about what you're trying to accomplish is supported.
vfio-pci only supports cooperative hotplug, and that's what it's trying
to implement here. The internal kernel PCI object is being torn down
even after the device has been physically removed, the PCI core is
trying to unbind it from the driver, which is where you're seeing the
device requests being relayed to the user. The user (QEMU or guest) is
probably hung up trying to access the device that no long exists to
respond to these unplug requests.
Finally, you've added the device back, but there's an entire chain of
policy decisions that needs to decide to bind that new device to
vfio-pci, decide that this guest should have access to that device, and
initiate a hot-add to the VM. That simply doesn't exist. Should this
guest still have access to the device at that bus address? Why? What
if it's an entirely new and different device? Who decides?
Someone needs to decide that this is a worthwhile feature to implement
and invest time to work out all these details before it "just works".
Perhaps you could share your use case to add weight to whether this is
something that should be pursued. The behavior you see is expected and
there is currently no ETA (or active development that I'm aware of) for
the behavior you desire. Thanks,
Alex
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