On 2025/11/13 12:17, Kasireddy, Vivek wrote:
Hi Akihiko,
Subject: Re: [PATCH v2 10/10] virtio-gpu-dmabuf: Create dmabuf for blobs
associated with VFIO devices
On 2025/11/12 13:26, Kasireddy, Vivek wrote:
Hi Akihiko,
Subject: Re: [PATCH v2 10/10] virtio-gpu-dmabuf: Create dmabuf for
blobs
associated with VFIO devices
On 2025/11/09 14:33, Vivek Kasireddy wrote:
In addition to memfd, a blob resource can also have its backing
storage in a VFIO device region. Therefore, we first need to figure
out if the blob is backed by a VFIO device region or a memfd before
we can call the right API to get a dmabuf fd created.
So, once we have the ramblock and the associated mr, we rely on
memory_region_is_ram_device() to tell us where the backing storage
is located. If the blob resource is VFIO backed, we try to find the
right VFIO device that contains the blob and then invoke the API
vfio_device_create_dmabuf().
Note that in virtio_gpu_remap_udmabuf(), we first try to test if
the VFIO dmabuf exporter supports mmap or not. If it doesn't, we
use the VFIO device fd directly to create the CPU mapping.
I have just remembered that mremap() will work for either of udmabuf
and
VFIO. That will avoid having two different methods and make
vfio_get_region_index_from_mr() and vfio_device_get_region_info()
unnecessary.
IIUC, the name virtio_gpu_remap_dmabuf() is misleading because we are
not
actually doing remap but are simply calling mmap(). In other words, we
are not
expanding or shrinking existing mapping but are creating a new mapping.
And, for dmabufs associated with VFIO devices, without having to call
vfio_get_region_index_from_mr() and vfio_device_get_region_info(), I
don't see
any other way to determine the region offset.
So, I guess I'll create a new patch to do s/remapped/map.
I mean calling mremap() with 0 as the old_size parameter. The man page
says:
> If the value of old_size is zero, and old_address refers to a
> shareable mapping (see the description of MAP_SHARED in mmap(2)),
then
> mremap() will create a new mapping of the same pages.
It might be possible to use mremap() here but using mmap() seems very
straightforward given that we are actually not shrinking or expanding
an existing mapping but are instead creating a new mapping. Also, I am
wondering what benefit would mremap() bring as opposed to just using
mmap()?
As I noted earlier, mremap() removes the need of having two different
paths for udmabuf and VFIO, and make vfio_get_region_index_from_mr() and
vfio_device_get_region_info() unnecessary, reducing code complexity.
mremap() is also sufficiently straightforward. The man page explicitly
states it is designed to create a new mapping. Using it for the purpose
(not shrinking or expanding an existing mapping) is not an abuse of the API.
Regards,
Akihiko Odaki
