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

[Chao Gao <chao.gao@intel.com>]

On Fri, Aug 09, 2019 at 10:49:32AM +0200, Jan Beulich wrote:
>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.

Yes. But it seems that no one is working on this feature now.

>
>>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?

No. Linux kernel guarantees that reset to a device won't affect
other devices. Otherwise, such device cannot be reset and no
'reset' attribute will be created under device's sysfs.
Specfically, the invocation of pci_dev_reset_slot_function() and
pci_parent_bus_reset() in pci_probe_reset_function() will check whether
the device (function) is the only one under the slot or bus
respectively. In pci_create_capabilities_sysfs(), 'reset' attribute is
created only if dev->reset_fn is not zero. 

Thanks
Chao

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