netdev - Re: [PATCH vhost v11 03/10] virtio_ring: introduce virtqueue_set

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Message-ID: <ZLjSj4uSr+pIKSui@infradead.org>
Date: Wed, 19 Jul 2023 23:22:07 -0700
From: Christoph Hellwig <hch@...radead.org>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Christoph Hellwig <hch@...radead.org>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	virtualization@...ts.linux-foundation.org,
	Jason Wang <jasowang@...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 03/10] virtio_ring: introduce
 virtqueue_set_premapped()

On Thu, Jul 13, 2023 at 10:47:23AM -0400, Michael S. Tsirkin wrote:
> There are a gazillion virtio drivers and most of them just use the
> virtio API, without bothering with these micro-optimizations.  virtio
> already tracks addresses so mapping/unmapping them for DMA is easier
> done in the core.  It's only networking and only with XDP where the
> difference becomes measureable.

Yes, but now you two differing code paths (which then branch into
another two with the fake DMA mappings).  I'm really worried about
the madness that follows like the USB dma mapping code that is a
constant soure of trouble.