netdev - Re: [PATCH vhost v4 00/10] virtio: drivers maintain dma info for premapped vq

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Message-ID: <1711009827.9194505-5-xuanzhuo@linux.alibaba.com>
Date: Thu, 21 Mar 2024 16:30:27 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: Jason Wang <jasowang@...hat.com>
Cc: virtualization@...ts.linux.dev,
 "Michael S. Tsirkin" <mst@...hat.com>,
 "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>,
 netdev@...r.kernel.org
Subject: Re: [PATCH vhost v4 00/10] virtio: drivers maintain dma info for premapped vq

On Thu, 21 Mar 2024 12:45:08 +0800, Jason Wang <jasowang@...hat.com> wrote:
> On Tue, Mar 12, 2024 at 11:36 AM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> >
> > As discussed:
> >
> > http://lore.kernel.org/all/CACGkMEvq0No8QGC46U4mGsMtuD44fD_cfLcPaVmJ3rHYqRZxYg@mail.gmail.com
> >
> > 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. We can release the memory used
> > to store the dma info.
> >
> > For virtio-net xmit queue, if the virtio-net maintains the dma info,
> > the virtio-net must allocate too much memory(19 * queue_size for per-queue), so
> > we do not plan to make the virtio-net to maintain the dma info by default. The
> > virtio-net xmit queue only maintain the dma info when premapped mode is enable
> > (such as AF_XDP is enable).
> >
> > So this patch set try to do:
> >
> > 1. make the virtio core to do not store the dma info
>
> I think you mean "make the virtio core to do not store the dma info
> when driver can do that"

YES.


>
> >     - 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.
>
> I guess you want to avoid allocating desc_extra array, otherwise you
> won't have this issue.
>
> How about keeping that?

This is a way. But when we allocate the indirect desc, we alloc
more memory to save that, I think that is a good way.
And in the future, we can handen the unmap for the indirect buffer
with more memory allocated by once.

Thanks.


>
> >       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 hardening
> >       the dma unmap is safe, we should store the dma info of indirect
> >       descs when the virtio core does not store the bufer dma info.
> >
> >       So I introduce the "structure the indirect desc table" to
> >       allocate space to store dma info of the desc table.
> >
> >         +struct vring_split_desc_indir {
> >         +       dma_addr_t addr;                /* Descriptor Array DMA addr. */
> >         +       u32 len;                        /* Descriptor Array length. */
> >         +       u32 num;
>
> We can probably just reuse vring_desc_extra here with a known flag
> (read only for device).
>
> >         +       struct vring_desc desc[];
> >         +};
> >
> >       The follow patches to this:
> >          * virtio_ring: packed: structure the indirect desc table
> >          * virtio_ring: split: structure the indirect desc table
> >
> >     - On the other side, in the umap handle, we mix the indirect descs with
> >       other descs. That make things too complex. I found if we we distinguish
> >       the descs with VRING_DESC_F_INDIRECT before unmap, thing will be clearer.
> >
> >       The follow patches do this.
> >          * virtio_ring: packed: remove double check of the unmap ops
> >          * virtio_ring: split: structure the indirect desc table
> >
> > 2. make the virtio core to enable premapped mode by find_vqs() params
> >     - Because the find_vqs() will try to allocate memory for the dma info.
> >       If we set the premapped mode after find_vqs() and release the
> >       dma info, that is odd.
>
> Thanks
>
> >
> >
> > Please review.
> >
> > Thanks
> >
> > v4:
> >     1. virtio-net xmit queue does not enable premapped mode by default
> >
> > v3:
> >     1. fix the conflict with the vp_modern_create_avq().
> >
> > 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 (10):
> >   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: add new parameter premapped
> >   virtio_ring: export premapped to driver by struct virtqueue
> >   virtio_net: set premapped mode by find_vqs()
> >   virtio_ring: virtqueue_set_dma_premapped support disable
> >
> >  drivers/net/virtio_net.c      |  57 +++--
> >  drivers/virtio/virtio_ring.c  | 436 +++++++++++++++++++++-------------
> >  include/linux/virtio.h        |   3 +-
> >  include/linux/virtio_config.h |  17 +-
> >  4 files changed, 307 insertions(+), 206 deletions(-)
> >
> > --
> > 2.32.0.3.g01195cf9f
> >
>