Re: [PATCH v8 0/5] Support message-based DMA in vfio-user server

[Peter Xu <peterx@redhat.com>]

On Mon, Mar 04, 2024 at 02:05:49AM -0800, Mattias Nissler wrote:
> This series adds basic support for message-based DMA in qemu's vfio-user
> server. This is useful for cases where the client does not provide file
> descriptors for accessing system memory via memory mappings. My motivating use
> case is to hook up device models as PCIe endpoints to a hardware design. This
> works by bridging the PCIe transaction layer to vfio-user, and the endpoint
> does not access memory directly, but sends memory requests TLPs to the hardware
> design in order to perform DMA.
> 
> Note that more work is needed to make message-based DMA work well: qemu
> currently breaks down DMA accesses into chunks of size 8 bytes at maximum, each
> of which will be handled in a separate vfio-user DMA request message. This is
> quite terrible for large DMA accesses, such as when nvme reads and writes
> page-sized blocks for example. Thus, I would like to improve qemu to be able to
> perform larger accesses, at least for indirect memory regions. I have something
> working locally, but since this will likely result in more involved surgery and
> discussion, I am leaving this to be addressed in a separate patch.

No objection from my side memory-wise.  It'll be good to get some words
from Paolo if possible.

Copy Peter Maydell due to the other relevant discussion.

https://lore.kernel.org/qemu-devel/20240228125939.56925-1-heinrich.schuchardt@canonical.com/

-- 
Peter Xu