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

["Teddy Astie" <teddy.astie@xxxxxxxxxx>]

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@xxxxxxxxxx>
>> Fixes: 61367688f1fb0 ("xen/virtio: enable grant based virtio on x86")
>> ---
>> CC: Juergen Gross <jgross@xxxxxxxx>
>> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>> CC: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>> CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
>>
>>   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.

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()).

>
> Juergen

Teddy


--
Teddy Astie | Vates XCP-ng Developer

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