Re: [Qemu-devel] virtIO question

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Wed, Nov 09, 2016 at 06:58:16PM +0800, zhunxun@gmail.com wrote:
> I want to ask a another question,why a virt_queue in virtio include in_sgs and out_sgs,for example,send_queue of virtIO net driver have in_sgs and out_sgs，when transmit data,It add buffer to out_sgs of send_queue,but how it to use in_sgs??

You can think of every virtqueue buffer as having two scatter-gather
lists:
1. out_sgs are driver->device buffers (e.g. tx packet payload)
2. in_sgs are device->driver buffers (e.g. rx packet payload)

Look at the virtio-net ctrl virtqueue (see spec and
virtio_net_handle_ctrl() for details).  Each buffer has:

1. struct virtio_net_ctrl_hdr (out_sgs)
2. request-specific fields (out_sgs)
3. virtio_net_ctrl_ack status byte (in_sgs)

The device parses the request and performs the operation.  Then it fills
in the result (success or error code) in the status byte.

Processing ctrl virtqueue buffers therefore requires both guest memory
reads (out_sgs) and writes (in_sgs).  Most of the other virtio devices
also use bi-directional buffers.

This may not be obvious if you only consider the virtio-net tx
virtqueue, for example, where buffers use out_sgs only.

Hope this makes sense.  If not, look at the specification again and
think about how virtio-net ctrl request processing works.

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