netdev - Re: [PATCH net-next] xsk: introduce xsk_dma

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Date:   Thu, 20 Apr 2023 09:18:21 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     Christoph Hellwig <hch@...radead.org>, netdev@...r.kernel.org,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        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>, bpf@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH net-next] xsk: introduce xsk_dma_ops

On Thu, Apr 20, 2023 at 05:11:48PM +0800, Xuan Zhuo wrote:
> I know that the current design of DMA API only supports some physical hardware,
> but can it be modified or expanded?

I think the important point is that for some cases there is no need
to dma map at all, and upper layers should be fine by that by just
doing the dma mapping in helpers called by the driver.

The virtio drivers then check if platform_access is set, then call the
generic dma mapping helper, or if not just allocate memory using
alloc_pages and also skip all the sync calls.