Re: [Qemu-devel] [PATCH 0/4] add failover feature for assigned network devices

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 21 May 2019 09:21:57 +0200
Jens Freimann <jfreimann@redhat.com> wrote:

> On Mon, May 20, 2019 at 04:56:57PM -0600, Alex Williamson wrote:
> >On Fri, 17 May 2019 14:58:16 +0200
> >Jens Freimann <jfreimann@redhat.com> wrote:  
> >> Command line example:
> >>
> >> qemu-system-x86_64 -enable-kvm -m 3072 -smp 3 \
> >>         -machine q35,kernel-irqchip=split -cpu host   \
> >>         -k fr   \
> >>         -serial stdio   \
> >>         -net none \
> >>         -qmp unix:/tmp/qmp.socket,server,nowait \
> >>         -monitor telnet:127.0.0.1:5555,server,nowait \
> >>         -device pcie-root-port,id=root0,multifunction=on,chassis=0,addr=0xa \
> >>         -device pcie-root-port,id=root1,bus=pcie.0,chassis=1 \
> >>         -device pcie-root-port,id=root2,bus=pcie.0,chassis=2 \
> >>         -netdev tap,script=/root/bin/bridge.sh,downscript=no,id=hostnet1,vhost=on \
> >>         -device virtio-net-pci,netdev=hostnet1,id=net1,mac=52:54:00:6f:55:cc,bus=root2,failover=on \
> >>         /root/rhel-guest-image-8.0-1781.x86_64.qcow2
> >>
> >> Then the primary device can be hotplugged via
> >>  (qemu) device_add vfio-pci,host=5e:00.2,id=hostdev0,bus=root1,standby=net1  
> >
> >Is this standby= option only valid for Network/Ethernet class code
> >devices?  If so, perhaps vfio-pci code should reject the option on any
> >non-ethernet devices.  The option is also non-intuitive for users, only
> >through examples like above can we see it relates to the id of the
> >secondary device.  Could we instead name it something like
> >"standby_net_failover_pair_id="?  
> 
> It is only for ethernet (VFs), I will add code to reject non-ethernet VF devices.
> I agree the name is not descriptive and the one you suggest seems good to
> me. 
> >
> >Also, this feature requires matching MAC addresses per the description,
> >where is that done?  Is it the user's responsibility to set the MAC on
> >the host device prior to the device_add?  If so, is this actually not
> >only specific to ethernet devices, but ethernet VFs?  
> 
> Yes, it's the users responsibility and the MACs are then matched by
> the net_failover driver in the guest. It makes sense for ethernet VFs only,
> I'll add a check for that.

FWIW, I'd probably stop at Ethernet class devices, vfio doesn't really
expose whether a device is a VF, so we'd likely need to resort to
getting that info through sysfs.  It also seems like there might be
some limited-use cases of copying the MAC from a PF to the virtio nic
or use of utilities on the host for modifiying a PF MAC, perhaps via
eeprom.  So while we expect the typical use case to be a VF, it's
probably ugly and unnecessarily restrictive to enforce it.  Thanks,

Alex