Re: [PATCH] xen/virtio: Don't use grant-dma-ops when running as Dom0

["Jürgen Groß" <jgross@suse.com>]

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On 06.01.26 23:47, Teddy Astie wrote:
> Le 06/01/2026 à 20:06, Jürgen Groß a écrit :
>> On 06.01.26 18:36, Teddy Astie wrote:
>>> Dom0 inherit devices from the machine and is usually in PV mode.
>>> If we are running in a virtual that has virtio devices, these devices
>>> would be considered as using grants with Dom0 as backend, while being
>>> the said Dom0 itself, while we want to use these devices like regular
>>> PCI devices.
>>>
>>> Fix this by preventing grant-dma-ops from being used when running as Dom0
>>> (initial domain). We still keep the device-tree logic as-is.
>>>
>>> Signed-off-by: Teddy Astie <teddy.astie@vates.tech>
>>> Fixes: 61367688f1fb0 ("xen/virtio: enable grant based virtio on x86")
>>> ---
>>> CC: Juergen Gross <jgross@suse.com>
>>> CC: Stefano Stabellini <sstabellini@kernel.org>
>>> CC: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>> CC: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>>>
>>>    drivers/xen/grant-dma-ops.c | 3 ++-
>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
>>> index 14077d23f2a1..c2603e700178 100644
>>> --- a/drivers/xen/grant-dma-ops.c
>>> +++ b/drivers/xen/grant-dma-ops.c
>>> @@ -366,7 +366,8 @@ static int xen_grant_init_backend_domid(struct
>>> device *dev,
>>>        if (np) {
>>>            ret = xen_dt_grant_init_backend_domid(dev, np, backend_domid);
>>>            of_node_put(np);
>>> -    } else if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) ||
>>> xen_pv_domain()) {
>>> +    } else if (!xen_initial_domain() &&
>>> +           (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) ||
>>> xen_pv_domain())) {
>>>            dev_info(dev, "Using dom0 as backend\n");
>>>            *backend_domid = 0;
>>>            ret = 0;
>>
>> Please make this controllable, e.g. via a boot parameter.
>>
>> It is completely valid to have a virtio device in dom0 with the backend in
>> a domU. You'll need grants in this case.
>>
> Due to
>   > *backend_domid = 0
> 
> Dom0 would always be the backend, unless we introduce a new boot
> parameter to select which domain will be the backend.

Ah, right. Okay, for now this is correct, so you can have my

Reviewed-by: Juergen Gross <jgross@suse.com>

In future things might change, though.

> There is also another issue, as in the xen_initial_domain() case, all
> PCI devices come from hardware. So no virtio-pci device can't come from
> another domain as Linux would pick up pcifront devices only if we are
> not a Dom0 (!xen_initial_domain()).

I think virtio devices should only be handled via pcifront in case they
are NOT implemented by a Xen backend domain.


Juergen

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