Subject: Re: [PATCH vhost v3 00/19] virtio: drivers maintain dma info for premapped vq

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 29 Feb 2024 03:21:14 -0500

On Thu, Feb 29, 2024 at 03:20:25PM +0800, Xuan Zhuo wrote:
> As discussed:
> http://lore.kernel.org/all/[email-masked]
> 
> If the virtio is premapped mode, the driver should manage the dma info by self.
> So the virtio core should not store the dma info.
> So we can release the memory used to store the dma info.
> 
> But if the desc_extra has not dma info, we face a new question,
> it is hard to get the dma info of the desc with indirect flag.
> For split mode, that is easy from desc, but for the packed mode,
> it is hard to get the dma info from the desc. And for hardening
> the dma unmap is saft, we should store the dma info of indirect
> descs.
> 
> So I introduce the "structure the indirect desc table" to
> allocate space to store dma info with the desc table.
> 
> On the other side, we mix the descs with indirect flag
> with other descs together to share the unmap api. That
> is complex. I found if we we distinguish the descs with
> VRING_DESC_F_INDIRECT before unmap, thing will be clearer.
> 
> Because of the dma array is allocated in the find_vqs(),
> so I introduce a new parameter to find_vqs().
> 
> Note:
>     this is on the top of
>         [PATCH vhost v1] virtio: packed: fix unmap leak for indirect desc table
>         http://lore.kernel.org/all/[email-masked]
> 
> Please review.
> 
> Thanks
> 
> v3:
>     1. fix the conflict with the vp_modern_create_avq().

Okay but are you going to address huge memory waste all this is causing for
- people who never do zero copy
- systems where dma unmap is a nop

?

You should address all comments when you post a new version, not just
what was expedient, or alternatively tag patch as RFC and explain
in commit log that you plan to do it later.

> v2:
>     1. change the dma item of virtio-net, every item have MAX_SKB_FRAGS + 2
>         addr + len pairs.
>     2. introduce virtnet_sq_free_stats for __free_old_xmit
> 
> v1:
>     1. rename transport_vq_config to vq_transport_config
>     2. virtio-net set dma meta number to (ring-size + 1)(MAX_SKB_FRGAS +2)
>     3. introduce virtqueue_dma_map_sg_attrs
>     4. separate vring_create_virtqueue to an independent commit
> 
> 
> 
> Xuan Zhuo (19):
>   virtio_ring: introduce vring_need_unmap_buffer
>   virtio_ring: packed: remove double check of the unmap ops
>   virtio_ring: packed: structure the indirect desc table
>   virtio_ring: split: remove double check of the unmap ops
>   virtio_ring: split: structure the indirect desc table
>   virtio_ring: no store dma info when unmap is not needed
>   virtio: find_vqs: pass struct instead of multi parameters
>   virtio: vring_create_virtqueue: pass struct instead of multi
>     parameters
>   virtio: vring_new_virtqueue(): pass struct instead of multi parameters
>   virtio_ring: simplify the parameters of the funcs related to
>     vring_create/new_virtqueue()
>   virtio: find_vqs: add new parameter premapped
>   virtio_ring: export premapped to driver by struct virtqueue
>   virtio_net: set premapped mode by find_vqs()
>   virtio_ring: remove api of setting vq premapped
>   virtio_ring: introduce dma map api for page
>   virtio_ring: introduce virtqueue_dma_map_sg_attrs
>   virtio_net: unify the code for recycling the xmit ptr
>   virtio_net: rename free_old_xmit_skbs to free_old_xmit
>   virtio_net: sq support premapped mode
> 
>  arch/um/drivers/virtio_uml.c             |  31 +-
>  drivers/net/virtio_net.c                 | 283 ++++++---
>  drivers/platform/mellanox/mlxbf-tmfifo.c |  24 +-
>  drivers/remoteproc/remoteproc_virtio.c   |  31 +-
>  drivers/s390/virtio/virtio_ccw.c         |  33 +-
>  drivers/virtio/virtio_mmio.c             |  30 +-
>  drivers/virtio/virtio_pci_common.c       |  59 +-
>  drivers/virtio/virtio_pci_common.h       |   9 +-
>  drivers/virtio/virtio_pci_legacy.c       |  16 +-
>  drivers/virtio/virtio_pci_modern.c       |  38 +-
>  drivers/virtio/virtio_ring.c             | 698 ++++++++++++-----------
>  drivers/virtio/virtio_vdpa.c             |  45 +-
>  include/linux/virtio.h                   |  13 +-
>  include/linux/virtio_config.h            |  48 +-
>  include/linux/virtio_ring.h              |  82 +--
>  tools/virtio/virtio_test.c               |   4 +-
>  tools/virtio/vringh_test.c               |  28 +-
>  17 files changed, 847 insertions(+), 625 deletions(-)
> 
> --
> 2.32.0.3.g01195cf9f