netdev - Re: [PATCH vhost v11 10/10] virtio_net: merge dma operation for one page

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Date: Thu, 20 Jul 2023 15:41:56 +0800
From: Jason Wang <jasowang@...hat.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, virtualization@...ts.linux-foundation.org, 
	"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>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>, netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH vhost v11 10/10] virtio_net: merge dma operation for one page

On Thu, Jul 20, 2023 at 2:23 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> Hi Jason,
>
> can you please resend your reply with proper quoting?  I had to give
> up after multiple pages of scrolling without finding anything that
> you added to the full quote.

I guess it's this part?

> > > You should also test without iommu but with swiotlb=force
> >
> >
> > For swiotlb, merge DMA has no benefit, because we still need to copy data from
> > swiotlb buffer to the origin buffer.
> > The benefit of the merge DMA is to reduce the operate to the iommu device.
> >
> > I did some test for this. The result is same.
> >
> > Thanks.
> >
>
> Did you actually check that it works though?
> Looks like with swiotlb you need to synch to trigger a copy
> before unmap, and I don't see where it's done in the current
> patch.

And this is needed for XDP_REDIRECT as well.

Thanks