netdev - Re: [PATCH net-next 04/13] virtio_ring: perform premapped operations based on per-buffer

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Message-ID: <CACGkMEvO+xg+237mq2Y+wDMxMum0CaiP3tMN81uqCGCfe_=Rbw@mail.gmail.com>
Date: Fri, 13 Sep 2024 11:36:32 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>, 
	Eugenio Pérez <eperezma@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>, virtualization@...ts.linux.dev, 
	bpf@...r.kernel.org
Subject: Re: [PATCH net-next 04/13] virtio_ring: perform premapped operations
 based on per-buffer

On Thu, Sep 12, 2024 at 3:43 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> On Wed, 11 Sep 2024 11:54:25 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > On Tue, Aug 20, 2024 at 3:33 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > >
> > > The current configuration sets the virtqueue (vq) to premapped mode,
> > > implying that all buffers submitted to this queue must be mapped ahead
> > > of time. This presents a challenge for the virtnet send queue (sq): the
> > > virtnet driver would be required to keep track of dma information for vq
> > > size * 17, which can be substantial. However, if the premapped mode were
> > > applied on a per-buffer basis, the complexity would be greatly reduced.
> > > With AF_XDP enabled, AF_XDP buffers would become premapped, while kernel
> > > skb buffers could remain unmapped.
> >
> > Is this only applied to TX or both TX and RX.
>
>
> For rx, if you mean per-buffer dma buffer, I think it is yes,
> rx can reuse this. If you mean should we do premapped for the
> normal rx buffers, I think we should, that can reduce the
> dma map operations.
>
>
> >
> > >
> > > We can distinguish them by sg_page(sg), When sg_page(sg) is NULL, this
> > > indicates that the driver has performed DMA mapping in advance, allowing
> > > the Virtio core to directly utilize sg_dma_address(sg) without
> > > conducting any internal DMA mapping.
> >
> > This seems conflict with the code below?
> >
> > #define sg_is_premapped(sg) (!sg_page(sg))
>
> Sorry, I do not get for you.
>
> The key point is that the sg->page is setted by driver.

Ok, I forget that but let's document this assumption in the changelog.

>
> So I mean if the driver sets sg->page = NULL, then for this sg,
> the virtio core can skip dma mapping. If the driver sets
> sg->page to the page of the buffer, then the virtio core should
> do dma mapping for this sg.
>

Ok, let's describe this in the changelog.

Thanks