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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.08.2019 10:38, Chao Gao wrote:
> 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.
Meanwhile I think a couple of years ago there was some initial effort
to get a newer chipset (Q35 iirc) emulated for HVM guests.

> 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?
Wouldn't a such emulated guest initiated reset affect other devices
(likely not under control of this guest) as well?

Jan

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