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

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Date:	Tue, 17 May 2011 08:22:14 -0700
From:	Shirley Ma <mashirle@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.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

On Tue, 2011-05-17 at 08:55 +0300, Michael S. Tsirkin wrote:
> Something in your patch that overwrites the id in vhost
> and makes it put the wrong id in the used ring?
> 
> By the way, need to keep in mind that a guest can
> give us the same head twice, need to make sure this
> at least does not corrupt host memory.

I think I didn't explain the problem very well here.

This patch doesn't overwrite the id. It just keeps the same coming
sequence from "head return vhost_get_vq_desc()" to pass to
vhost_add_used.

The same ids can be used many times once it passes to guest from
vhost_add_used. There is no problem. The zero copy patch doesn't have
any issue.

The problem is the order of head from return vhost_get_vq_desc should be
in sequence when it passes to vhost_add_used.

The original code has no problem, because it gets one head and pass that
head to vhost_add_used one by one once done the copy. So it's in
sequence.

This issue can easily recreate without zerocopy patch by simply changing
the order from "head return vhost_get_vq_desc" when passing to
vhost_add_used.

Thanks
Shirley

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