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

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Message-ID: <995f7ac8-0a8b-43d9-9cc7-63622ec52ca1@suse.com>
Date: Tue, 6 Jan 2026 20:03:39 +0100
From: Jürgen Groß <jgross@...e.com>
To: Teddy Astie <teddy.astie@...es.tech>, xen-devel@...ts.xenproject.org,
 linux-kernel@...r.kernel.org
Cc: Stefano Stabellini <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH] xen/virtio: Don't use grant-dma-ops when running as Dom0

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@...es.tech>
> Fixes: 61367688f1fb0 ("xen/virtio: enable grant based virtio on x86")
> ---
> CC: Juergen Gross <jgross@...e.com>
> CC: Stefano Stabellini <sstabellini@...nel.org>
> CC: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
> CC: Boris Ostrovsky <boris.ostrovsky@...cle.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.


Juergen

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