linux-kernel - Re: [PATCHv11] vhost: vhost TX zero-copy support

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Date:	Mon, 18 Jul 2011 10:43:21 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mst@...hat.com
Cc:	mashirle@...ibm.com, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, jj@...osbits.net
Subject: Re: [PATCHv11] vhost: vhost TX zero-copy support

From: "Michael S. Tsirkin" <mst@...hat.com>
Date: Mon, 18 Jul 2011 16:48:46 +0300

>>From: Shirley Ma <mashirle@...ibm.com>
> 
> This adds experimental zero copy support in vhost-net,
> disabled by default. To enable, set
> experimental_zcopytx module option to 1.
> 
> This patch maintains the outstanding userspace buffers in the
> sequence it is delivered to vhost. The outstanding userspace buffers
> will be marked as done once the lower device buffers DMA has finished.
> This is monitored through last reference of kfree_skb callback. Two
> buffer indices are used for this purpose.
> 
> The vhost-net device passes the userspace buffers info to lower device
> skb through message control. DMA done status check and guest
> notification are handled by handle_tx: in the worst case is all buffers
> in the vq are in pending/done status, so we need to notify guest to
> release DMA done buffers first before we get any new buffers from the
> vq.
> 
> One known problem is that if the guest stops submitting
> buffers, buffers might never get used until some
> further action, e.g. device reset. This does not
> seem to affect linux guests.
> 
> Signed-off-by: Shirley <xma@...ibm.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>

Applied, thanks!
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