Re: [RFC PATCH] virtio-mmio: add xenbus probing

[Val Packett <val@invisiblethingslab.com>]

On 4/29/26 12:35 PM, Jürgen Groß wrote:
> Some minor details from the Xen side of things:
>
> On 29.04.26 15:52, Val Packett wrote:
>> The experimental virtio-mmio support for Xen was initially developed
>> on aarch64, so device trees were used to configure the mmio devices,
>> with arbitrary vGIC interrupts used by the hypervisor. On x86_64
>> however, the only reasonable way to interrupt the guest is over Xen
>> event channels, which can only be acquired by children of xenbus,
>
> More exact: interdomain event channels need to be connected to a xenbus
> device. But you are needing those, so for your use case the above 
> statement
> is correct.
>
>> the virtual bus driven by Xen's configuration database, XenStore.
>> It is also a more convenient and "Xen-ish" way to provision devices.
>>
>> Implement a xenbus client for virtio-mmio which negotiates an
>> event channel and provides it as a platform IRQ to the
>> virtio-mmio driver.
>>
>>
>> Signed-off-by: Val Packett <val@invisiblethingslab.com>
>> ---
>>
>> Hi,
>>
>> I've been working on porting virtio-mmio support from Arm to x86_64,
>> with the goal of running vhost-user-gpu to power Wayland/GPU integration
>> for Qubes OS. (I'm aware of various proposals for alternative virtio
>> transports but virtio-mmio seems to be the only one that *is* upstream
>> already and just Works..) Setting up virtio-mmio through xenbus, 
>> initially
>> motivated just by event channels being the only real way to get 
>> interrupts
>> working on HVM, turned out to generally be quite pleasant and nice :)
>>
>> I'd like to get some early feedback for this patch, particularly
>> the general stuff:
>>
>> * is this whole thing acceptable in general?
>> * should it be extracted into a different file?
>> * (from the Xen side) any input on the xenstore keys, what goes where?
>
> You should add some documentation in the Xen source tree regarding the
> Xenstore keys (see docs/misc/xenstore-paths.pandoc there).
Ack, thanks!
>> […]
>>
>> +again:
>> +    err = xenbus_transaction_start(&xbt);
>
> No need to use a Xenstore transaction here. The written node(s) are
> regarded to be valid only after calling xenbus_switch_state() to set
> the frontend state to XenbusStateInitialised.
Oh, I assumed transactions were required for writing from the kernel to 
work at all…
>> [..]
>>
>> +static const struct xenbus_device_id virtio_mmio_xen_ids[] = {
>> +    { "virtio" },
>
> Please use "virtio-mmio" here, as I could imagine "virtio-pci" 
> devices, too. 

Ack. Would actually also distinguish it from the initial Arm 
proof-of-concept version…


Thanks,
~val