linux-kernel - Re: [PATCH v12 13/13] Documentation: Add documentation for VDUSE Address Space IDs

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Message-ID: <CAJaqyWcz4+ixvUJ_brK5k4dRVdp9dHP-LT30hEc0mFNjQDRp4Q@mail.gmail.com>
Date: Fri, 16 Jan 2026 12:23:41 +0100
From: Eugenio Perez Martin <eperezma@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Maxime Coquelin <mcoqueli@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	virtualization@...ts.linux.dev, jasowang@...hat.com, 
	Stefano Garzarella <sgarzare@...hat.com>, Yongji Xie <xieyongji@...edance.com>, 
	linux-kernel@...r.kernel.org, Cindy Lu <lulu@...hat.com>, 
	Laurent Vivier <lvivier@...hat.com>
Subject: Re: [PATCH v12 13/13] Documentation: Add documentation for VDUSE
 Address Space IDs

On Thu, Jan 15, 2026 at 9:41 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Wed, Jan 14, 2026 at 07:48:39PM +0100, Eugenio Pérez 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 passtrhough to the guest
>
> passthrough
>
> > 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>
> > ---
> > v12: New in V12. Requested by Jason.
> > ---
> >  Documentation/userspace-api/vduse.rst | 49 +++++++++++++++++++++++++++
> >  1 file changed, 49 insertions(+)
> >
> > diff --git a/Documentation/userspace-api/vduse.rst b/Documentation/userspace-api/vduse.rst
> > index bdb880e01132..66110d918815 100644
> > --- a/Documentation/userspace-api/vduse.rst
> > +++ b/Documentation/userspace-api/vduse.rst
> > @@ -230,4 +230,53 @@ 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. Setup it
>
> 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_IOTLB_GET_INFO) to
> > +select the address space of the IOTLB you are queryng.
>
> querying
>
> > Similarly, you can use
> > +ioctl(VDUSE_IOTLB_GET_FD2) to obtain the file descriptor describing an IOVA
> > +region of an specific ASID. Example usage:
>
> a specific
>
> > +
> > +.. 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.

Fixing typos for the next version, thanks!