netdev - Re: [PATCH V5 4/6 net-next] vhost: vhost TX zero-copy support

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Message-ID: <20110516204540.GD18148@redhat.com>
Date:	Mon, 16 May 2011 23:45:40 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Avi Kivity <avi@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [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.

-- 
MST
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