netdev - Re: [PATCH vhost v3 0/4] virtio_net: rx enable premapped mode by default

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Date: Sun, 28 Apr 2024 09:18:49 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>,
 Jason Wang <jasowang@...hat.com>,
 "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>,
 netdev@...r.kernel.org,
 virtualization@...ts.linux.dev
Subject: Re: [PATCH vhost v3 0/4] virtio_net: rx enable premapped mode by default

On Fri, 26 Apr 2024 13:00:08 +0200, Paolo Abeni <pabeni@...hat.com> wrote:
> On Wed, 2024-04-24 at 16:16 +0800, Xuan Zhuo wrote:
> > Actually, for the virtio drivers, we can enable premapped mode whatever
> > the value of use_dma_api. Because we provide the virtio dma apis.
> > So the driver can enable premapped mode unconditionally.
> >
> > This patch set makes the big mode of virtio-net to support premapped mode.
> > And enable premapped mode for rx by default.
> >
> > Based on the following points, we do not use page pool to manage these
> >     pages:
> >
> >     1. virtio-net uses the DMA APIs wrapped by virtio core. Therefore,
> >        we can only prevent the page pool from performing DMA operations, and
> >        let the driver perform DMA operations on the allocated pages.
> >     2. But when the page pool releases the page, we have no chance to
> >        execute dma unmap.
> >     3. A solution to #2 is to execute dma unmap every time before putting
> >        the page back to the page pool. (This is actually a waste, we don't
> >        execute unmap so frequently.)
> >     4. But there is another problem, we still need to use page.dma_addr to
> >        save the dma address. Using page.dma_addr while using page pool is
> >        unsafe behavior.
> >     5. And we need space the chain the pages submitted once to virtio core.
> >
> >     More:
> >         https://lore.kernel.org/all/CACGkMEu=Aok9z2imB_c5qVuujSh=vjj1kx12fy9N7hqyi+M5Ow@mail.gmail.com/
> >
> > Why we do not use the page space to store the dma?
> >     http://lore.kernel.org/all/CACGkMEuyeJ9mMgYnnB42=hw6umNuo=agn7VBqBqYPd7GN=+39Q@mail.gmail.com
> >
> > Please review.
> >
> > v3:
> >     1. big mode still use the mode that virtio core does the dma map/unmap
> >
> > v2:
> >     1. make gcc happy in page_chain_get_dma()
> >         http://lore.kernel.org/all/202404221325.SX5ChRGP-lkp@intel.com
> >
> > v1:
> >     1. discussed for using page pool
> >     2. use dma sync to replace the unmap for the first page
>
> Judging by the subj prefix, this is targeting the vhost tree, right?
>
> There are a few patches landing on virtio_net on net-next, I guess
> there will be some conflict while pushing to Linux (but I haven't
> double check yet!)
>
> Perhaps you could provide a stable git branch so that both vhost and
> netdev could pull this set?

This patch set is related with the virio core, so I push this
to the vhost branch. And there is no new feature to the virtio-net.
Similar situation in the past, we pushed to the vhost branch.

I am ok to net-next or vhost. We can hear others

@Michael

Thanks.

>
> Thanks!
>
> Paolo
>