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

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Message-ID: <ZE89E+qJgGPNUjT5@infradead.org>
Date:   Sun, 30 Apr 2023 21:16:19 -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>, 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,
        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 Tue, Apr 25, 2023 at 04:12:05AM -0400, Michael S. Tsirkin wrote:
> In theory, absolutely. In practice modern virtio devices are ok,
> the reason we are stuck supporting old legacy ones is because legacy
> devices are needed to run old guests. And then people turn
> around and run a new guest on the same device,
> for example because they switch back and forth e.g.
> for data recovery? Or because whoever is selling the
> host wants to opt for maximum compatibility.
> 
> Teaching all of linux to sometimes use dma and sometimes not
> is a lot of work, and for limited benefit of these legacy systems.

It's not like virtio is the only case where blindly assuming
your can use DMA operations in a higher layer is the problem..