linux-kernel - Re: [PATCH v14 13/13] Documentation: Add documentation for VDUSE Address Space IDs

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Message-ID: <CACGkMEthnMXVYDEO+=XPumyWf9abN8bCLVQ42emYOWZQuDLG-w@mail.gmail.com>
Date: Mon, 19 Jan 2026 15:19:26 +0800
From: Jason Wang <jasowang@...hat.com>
To: Eugenio Pérez <eperezma@...hat.com>
Cc: "Michael S . Tsirkin" <mst@...hat.com>, Laurent Vivier <lvivier@...hat.com>, linux-kernel@...r.kernel.org, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Maxime Coquelin <mcoqueli@...hat.com>, 
	Cindy Lu <lulu@...hat.com>, virtualization@...ts.linux.dev, 
	Yongji Xie <xieyongji@...edance.com>, Stefano Garzarella <sgarzare@...hat.com>
Subject: Re: [PATCH v14 13/13] Documentation: Add documentation for VDUSE
 Address Space IDs

On Fri, Jan 16, 2026 at 10:05 PM Eugenio Pérez <eperezma@...hat.com> wrote:
>
> Address Space IDs allows the VDUSE framework to support devices able to
> expose different virtqueues to different part of the drivers.  For
> example, to let QEMU handle the net device control virtqueue, so QEMU
> always knows the state of the device like mac address or number of
> queues enabled, while leaving the dataplane passthrough to the guest
> intact.  This enables live migration.
>
> Expands the VDUSE documentation to explain how to use the new ioctls or
> the new struct members of old ioctls.
>
> Signed-off-by: Eugenio Pérez <eperezma@...hat.com>
> ---
> v13:
> * Fix s/VDUSE_IOTLB_GET_INFO/VDUSE_VQ_SETUP/.
> * Document VDUSE_SET_VQ_GROUP_ASID VDUSE message (Jason).
> * Fix typos (MST and Jason).
>
> v12: New in V12. Requested by Jason.
> ---
>  Documentation/userspace-api/vduse.rst | 53 +++++++++++++++++++++++++++
>  1 file changed, 53 insertions(+)
>
> diff --git a/Documentation/userspace-api/vduse.rst b/Documentation/userspace-api/vduse.rst
> index bdb880e01132..aa067da92cf5 100644
> --- a/Documentation/userspace-api/vduse.rst
> +++ b/Documentation/userspace-api/vduse.rst
> @@ -230,4 +230,57 @@ able to start the dataplane processing as follows:
>  5. Inject an interrupt for specific virtqueue with the VDUSE_INJECT_VQ_IRQ ioctl
>     after the used ring is filled.
>
> +Enabling ASID (API version 1)
> +------------------------------
> +
> +VDUSE supports per-address-space identifiers (ASIDs) starting with API
> +version 1. Set it up with ioctl(VDUSE_SET_API_VERSION) on `/dev/vduse/control`
> +and pass `VDUSE_API_VERSION_1` before creating a new VDUSE instance with
> +ioctl(VDUSE_CREATE_DEV).
> +
> +Afterwards, you can use the member asid of ioctl(VDUSE_VQ_SETUP) argument to
> +select the address space of the IOTLB you are querying.  The driver could
> +change the address space of any virtqueue group by using the
> +VDUSE_SET_VQ_GROUP_ASID VDUSE message type, and the VDUSE instance needs to
> +reply with VDUSE_REQ_RESULT_OK if it was possible to change it.

Nitpick: Is this better to move this to the part of

"There are now three types of messages introduced by VDUSE framework:"

?

> +
> +Similarly, you can use ioctl(VDUSE_IOTLB_GET_FD2) to obtain the file descriptor
> +describing an IOVA region of a specific ASID. Example usage:
> +
> +.. code-block:: c
> +
> +       static void *iova_to_va(int dev_fd, uint32_t asid, uint64_t iova,
> +                               uint64_t *len)
> +       {
> +               int fd;
> +               void *addr;
> +               size_t size;
> +               struct vduse_iotlb_entry_v2 entry = { 0 };
> +
> +               entry.v1.start = iova;
> +               entry.v1.last = iova;
> +               entry.asid = asid;
> +
> +               fd = ioctl(dev_fd, VDUSE_IOTLB_GET_FD2, &entry);
> +               if (fd < 0)
> +                       return NULL;
> +
> +               size = entry.v1.last - entry.v1.start + 1;
> +               *len = entry.v1.last - iova + 1;
> +               addr = mmap(0, size, perm_to_prot(entry.v1.perm), MAP_SHARED,
> +                           fd, entry.v1.offset);
> +               close(fd);
> +               if (addr == MAP_FAILED)
> +                       return NULL;
> +
> +               /*
> +                * Using some data structures such as linked list to store
> +                * the iotlb mapping. The munmap(2) should be called for the
> +                * cached mapping when the corresponding VDUSE_UPDATE_IOTLB
> +                * message is received or the device is reset.
> +                */
> +
> +               return addr + iova - entry.start;
> +       }
> +
>  For more details on the uAPI, please see include/uapi/linux/vduse.h.
> --
> 2.52.0
>

Thanks