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SubjectRe: [PATCH V5 4/6 net-next] vhost: vhost TX zero-copy support
> +/* Since we need to keep the order of used_idx as avail_idx, it's possible that
> + * DMA done not in order in lower device driver for some reason. To prevent
> + * used_idx out of order, upend_idx is used to track avail_idx order, done_idx
> + * is used to track used_idx order. Once lower device DMA done, then upend_idx
> + * can move to done_idx.

Could you clarify this please? virtio explicitly allows out of order
completion of requests. Does it simplify code that we try to keep
used index updates in-order? Because if not, this is not
really a requirement.


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