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[Xen-devel] Reset pass-thru devices in a VM

Hi everyone,

I have a device which only supports secondary bus reset. After being
assigned to a VM, it would be placed under host bridge. For devices
under host bridge, secondary bus reset is not applicable. Thus, a VM
has no way to reset this device.

This device's usage would be limited without PCI reset (for example, its
driver cannot re-initialize the device properly without PCI reset, which
means in VM device won't be usable after unloading the driver), it would
be much better if there is a way available to VMs to reset the device.

In my mind, a straightfoward solution is to create a virtual bridge
for a VM and place the pass-thru device under a virtual bridge. But it
isn't supported in Xen (KVM/QEMU supports) and enabling it looks need
a lot of efforts. Alternatively, emulating FLR (Function Level Reset)
capability for this device might be a feasible way and only needs
relatively few changes. I am planning to enable an opt-in feature
(like 'permissive') to allow qemu to expose FLR capability to guest for
pass-thru devices as long as this device is resetable on dom0 (i.e. the
device has 'reset' attribute under its sysfs). And when guest initiates
an FLR, qemu just echo 1 to the 'reset' attribute on dom0.

Do you think emulating FLR capability is doable?


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