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

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Date: Thu, 13 Jul 2023 04:14:45 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: virtualization@...ts.linux-foundation.org,
	"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>, 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, Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH vhost v11 03/10] virtio_ring: introduce
 virtqueue_set_premapped()

On Mon, Jul 10, 2023 at 11:42:30AM +0800, Xuan Zhuo wrote:
> This helper allows the driver change the dma mode to premapped mode.
> Under the premapped mode, the virtio core do not do dma mapping
> internally.
> 
> This just work when the use_dma_api is true. If the use_dma_api is false,
> the dma options is not through the DMA APIs, that is not the standard
> way of the linux kernel.

I have a hard time parsing this.

More importantly having two modes seems very error prone going down
the route.  If the premapping is so important, why don't we do it
always?