netdev - RE: issue about virtio-net driver to suppoprt vhost mergeable buffer with zero-copy to support PS mode

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Date:	Fri, 28 May 2010 09:13:29 +0800
From:	"Xin, Xiaohui" <xiaohui.xin@...el.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Herbert Xu <herbert@...dor.hengli.com.au>
Subject: RE: issue about virtio-net driver to suppoprt vhost mergeable
	buffer with zero-copy to support PS mode

Michael,
What's you have suggested could avoid to taint the guest virtio-net driver. Really thanks!
For the two alternative, the first will do more trick with driver or net-core stuff. So currently, I prefer to try the second one. Anyway, let me have a good think of it. Thanks!

Thanks
Xiaohui

>-----Original Message-----
>From: kvm-owner@...r.kernel.org [mailto:kvm-owner@...r.kernel.org] On Behalf Of
>Michael S. Tsirkin
>Sent: Thursday, May 27, 2010 4:20 PM
>To: Xin, Xiaohui
>Cc: netdev@...r.kernel.org; kvm@...r.kernel.org; linux-kernel@...r.kernel.org; Herbert
>Xu
>Subject: Re: issue about virtio-net driver to suppoprt vhost mergeable buffer with zero-copy
>to support PS mode
>
>On Thu, May 27, 2010 at 09:21:02AM +0800, Xin, Xiaohui wrote:
>> Michael,
>> I'm now looking into the vhost mergeable buffer, and I tried to use it to support PS mode
>with zero-copy. And I found an issue there that I have to modify the guest virito-net driver.
>>
>> When guest virtio-net driver submits mergeable buffers, it submits multiple pages outside.
>In zero-copy case, vhost cannot know which page is used to put header, and which page is
>used to put payload. Then vhost can only reserves 12 bytes for each page. That means, the
>page_offset of the payload DMAed into the guest buffer is always 12 bytes. But guest
>virtio-net driver always use offset 0 to put the data (See receive_mergeable()). That's where
>the zero-copy use mergeable buffer must modify.
>>
>> Have I missed something here? And how do you think about it?
>>
>> Thanks
>> Xiaohui
>
>Maybe you can teach the hardware skip the first 12 bytes: qemu will
>call an ioctl telling hardware what the virtio header size is.
>This is how we plan to do it for tap.
>
>Alternatively, buffers can be used in any order.
>So we can have hardware use N buffers for the packet, and then
>have vhost put the header in buffer N+1.
>
>--
>MST
>--
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