Re: [PATCH 2/6] virtio: split: alloc indirect desc with extra

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jan 10, 2022 at 05:23:13PM +0800, Xuan Zhuo wrote:
> On Mon, 10 Jan 2022 16:54:58 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > On Mon, Jan 10, 2022 at 3:59 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > >
> > > On Mon, 10 Jan 2022 15:41:27 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > > On Mon, Jan 10, 2022 at 3:24 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > > > >
> > > > > On Mon, 10 Jan 2022 14:43:39 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > > > >
> > > > > > 在 2022/1/7 下午2:33, Xuan Zhuo 写道:
> > > > > > > In the scenario where indirect is not used, each desc corresponds to an
> > > > > > > extra, which is used to record information such as dma, flags, and
> > > > > > > next.
> > > > > > >
> > > > > > > In the scenario of using indirect, the assigned desc does not have the
> > > > > > > corresponding extra record dma information, and the dma information must
> > > > > > > be obtained from the desc when unmap.
> > > > > > >
> > > > > > > This patch allocates the corresponding extra array when indirect desc is
> > > > > > > allocated. This has these advantages:
> > > > > > > 1. Record the dma information of desc, no need to read desc when unmap
> > > > > > > 2. It will be more convenient and unified in processing
> > > > > > > 3. Some additional information can be recorded in extra, which will be
> > > > > > >     used in subsequent patches.
> > > > > >
> > > > > >
> > > > > > Two questions:
> > > > > >
> > > > > > 1) Is there any performance number for this change? I guess it gives
> > > > > > more stress on the cache.
> > > > >
> > > > > I will add performance test data in the next version.
> > > > >
> > > > > > 2) Is there a requirement to mix the pre mapped sg with unmapped sg? If
> > > > > > not, a per virtqueue flag looks sufficient
> > > > >
> > > > > There is this requirement. For example, in the case of AF_XDP, a patcket
> > > > > contains two parts, one is virtio_net_hdr, and the other is the actual data
> > > > > packet from AF_XDP. The former is unmapped sg, and the latter is pre mapped sg.
> > > >
> > > > Any chance to map virtio_net_hdr() manually by AF_XDP routine in this case?
> > >
> > > Well, it is indeed possible to do so. In the indirect scenario, we can record it
> > > in vring->split.desc_extra[head].flags
> > >
> > > Then we have to agree that there can be no mixed situation.
> >
> > I think it would be easier and less performance regression if we don't
> > do huge changes in the core unless it's a must.
> >
> 
> Ok, I plan to add two new interface virtqueue_add_outbuf_flag(),
> virtqueue_add_inbuf_flag() pass a flag parameter to virtqueue_add() to
> mark sgs addr is predma.

I would maybe just do a variant working with mapped SGs.

> I don't want to use sg->dma_address, so we have to check whether each sg uses
> dma_address. If it is not unified, we will also handle exception.
> 
> > Btw, I forgot the conclusion of the last AF_XDP series. Why is it
> > better to change virtio_ring instead of AF_XDP (which seems easier).
> 
> Regarding this question, I'm guessing you mean to make AF_XDP not use DMA
> addresses? Instead pass virtual addresses to virtio.
> 
> It would certainly be simpler, but I think there is a performance gain in doing
> the DMA mapping ahead of time.
> 
> Thanks.

That would at least in part depend on whether DMA mapping is eventually used ;)
Testing the actual gain before we introduce complexity and including
that info in the commit log is not a bad idea.

> 
> >
> > Thanks
> >
> > >
> > > Thanks.
> > >
> > > >
> > > > Thanks
> > > >
> > > > >
> > > > > Thanks.
> > > > >
> > > > > >
> > > > > > Thanks
> > > > > >
> > > > > >
> > > > >
> > > >
> > >
> >

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization