Re: [PATCH 0/3] Support message-based DMA in vfio-user server

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Jul 04, 2023 at 01:06:24AM -0700, 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 in addition to the 3 commits included, we also need a
> subprojects/libvfio-user roll to bring in this bugfix:
> https://github.com/nutanix/libvfio-user/commit/bb308a2e8ee9486a4c8b53d8d773f7c8faaeba08
> Stefan, can I ask you to kindly update the
> https://gitlab.com/qemu-project/libvfio-user mirror? I'll be happy to include
> an update to subprojects/libvfio-user.wrap in this series.

Done:
https://gitlab.com/qemu-project/libvfio-user/-/commits/master

Repository mirroring is automated now, so new upstream commits will
appear in the QEMU mirror repository from now on.

> 
> Finally, there is some more work required on top of this series to get
> message-based DMA to really work well:
> 
> * libvfio-user has a long-standing issue where socket communication gets messed
>   up when messages are sent from both ends at the same time. See
>   https://github.com/nutanix/libvfio-user/issues/279 for more details. I've
>   been engaging there and plan to contribute a fix.
> 
> * 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 acceses, 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.
> 
> Mattias Nissler (3):
>   softmmu: Support concurrent bounce buffers
>   softmmu: Remove DMA unmap notification callback
>   vfio-user: Message-based DMA support
> 
>  hw/remote/vfio-user-obj.c |  62 ++++++++++++++++--
>  softmmu/dma-helpers.c     |  28 --------
>  softmmu/physmem.c         | 131 ++++++++------------------------------
>  3 files changed, 83 insertions(+), 138 deletions(-)

Sorry for the late review. I was on vacation and am catching up on
emails.

Paolo worked on the QEMU memory API and can give input on how to make
this efficient for large DMA accesses. There is a chance that memory
dispatch with larger sizes will be needed for ENQCMD CPU instruction
emulation too.

Stefan

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