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Date:	Thu, 15 Apr 2010 17:36:07 +0800
From:	"Xin, Xiaohui" <>
To:	"Michael S. Tsirkin" <>
CC:	"" <>,
	"" <>,
	"" <>,
	"" <>,
	"" <>,
	"" <>
Subject: RE: [RFC][PATCH v2 0/3] Provide a zero-copy method on KVM

>> The idea is simple, just to pin the guest VM user space and then
>> let host NIC driver has the chance to directly DMA to it. 
>> The patches are based on vhost-net backend driver. We add a device
>> which provides proto_ops as sendmsg/recvmsg to vhost-net to
>> send/recv directly to/from the NIC driver. KVM guest who use the
>> vhost-net backend may bind any ethX interface in the host side to
>> get copyless data transfer thru guest virtio-net frontend.
>> The scenario is like this:
>> The guest virtio-net driver submits multiple requests thru vhost-net
>> backend driver to the kernel. And the requests are queued and then
>> completed after corresponding actions in h/w are done.
>> For read, user space buffers are dispensed to NIC driver for rx when
>> a page constructor API is invoked. Means NICs can allocate user buffers
>> from a page constructor. We add a hook in netif_receive_skb() function
>> to intercept the incoming packets, and notify the zero-copy device.
>> For write, the zero-copy deivce may allocates a new host skb and puts
>> payload on the skb_shinfo(skb)->frags, and copied the header to skb->data.
>> The request remains pending until the skb is transmitted by h/w.
>> Here, we have ever considered 2 ways to utilize the page constructor
>> API to dispense the user buffers.
>> One:	Modify __alloc_skb() function a bit, it can only allocate a 
>> 	structure of sk_buff, and the data pointer is pointing to a 
>> 	user buffer which is coming from a page constructor API.
>> 	Then the shinfo of the skb is also from guest.
>> 	When packet is received from hardware, the skb->data is filled
>> 	directly by h/w. What we have done is in this way.
>> 	Pros:	We can avoid any copy here.
>> 	Cons:	Guest virtio-net driver needs to allocate skb as almost
>> 		the same method with the host NIC drivers, say the size
>> 		of netdev_alloc_skb() and the same reserved space in the
>> 		head of skb. Many NIC drivers are the same with guest and
>> 		ok for this. But some lastest NIC drivers reserves special
>> 		room in skb head. To deal with it, we suggest to provide
>> 		a method in guest virtio-net driver to ask for parameter
>> 		we interest from the NIC driver when we know which device 
>> 		we have bind to do zero-copy. Then we ask guest to do so.
>> 		Is that reasonable?

>Unfortunately, this would break compatibility with existing virtio.
>This also complicates migration. 

You mean any modification to the guest virtio-net driver will break the
compatibility? We tried to enlarge the virtio_net_config to contains the
2 parameter, and add one VIRTIO_NET_F_PASSTHRU flag, virtionet_probe()
will check the feature flag, and get the parameters, then virtio-net driver use
it to allocate buffers. How about this?

>What is the room in skb head used for?
I'm not sure, but the latest ixgbe driver does this, it reserves 32 bytes compared to

>> Two:	Modify driver to get user buffer allocated from a page constructor
>> 	API(to substitute alloc_page()), the user buffer are used as payload
>> 	buffers and filled by h/w directly when packet is received. Driver
>> 	should associate the pages with skb (skb_shinfo(skb)->frags). For 
>> 	the head buffer side, let host allocates skb, and h/w fills it. 
>> 	After that, the data filled in host skb header will be copied into
>> 	guest header buffer which is submitted together with the payload buffer.
>> 	Pros:	We could less care the way how guest or host allocates their
>> 		buffers.
>> 	Cons:	We still need a bit copy here for the skb header.
>> We are not sure which way is the better here.

>The obvious question would be whether you see any speed difference
>with the two approaches. If no, then the second approach would be

I remember the second approach is a bit slower in 1500MTU. 
But we did not tested too much.

>> This is the first thing we want
>> to get comments from the community. We wish the modification to the network
>> part will be generic which not used by vhost-net backend only, but a user
>> application may use it as well when the zero-copy device may provides async
>> read/write operations later.
>> Please give comments especially for the network part modifications.
>> We provide multiple submits and asynchronous notifiicaton to 
>>vhost-net too.
>> Our goal is to improve the bandwidth and reduce the CPU usage.
>> Exact performance data will be provided later. But for simple
>> test with netperf, we found bindwidth up and CPU % up too,
>> but the bindwidth up ratio is much more than CPU % up ratio.
>> What we have not done yet:
>> 	packet split support

>What does this mean, exactly?
We can support 1500MTU, but for jumbo frame, since vhost driver before don't 
support mergeable buffer, we cannot try it for multiple sg. A jumbo frame will split 5
frags and hook them once a descriptor, so the user buffer allocation is greatly dependent
on how guest virtio-net drivers submits buffers. We think mergeable buffer is suitable for it. 

>> 	To support GRO
Actually, I think if the mergeable buffer may get good performance, then GRO is not 
so important then.
Do we really need them?

>> 	Performance tuning
>> what we have done in v1:
>> 	polish the RCU usage
>> 	deal with write logging in asynchroush mode in vhost
>> 	add notifier block for mp device
>> 	rename page_ctor to mp_port in netdevice.h to make it looks generic
>> 	add mp_dev_change_flags() for mp device to change NIC state
>> 	add CONIFG_VHOST_MPASSTHRU to limit the usage when module is not load
>> 	a small fix for missing dev_put when fail
>> 	using dynamic minor instead of static minor number
>> 	a __KERNEL__ protect to mp_get_sock()
>> what we have done in v2:
>> 	remove most of the RCU usage, since the ctor pointer is only
>> 	changed by BIND/UNBIND ioctl, and during that time, NIC will be
>> 	stopped to get good cleanup(all outstanding requests are finished),
>> 	so the ctor pointer cannot be raced into wrong situation.
>> 	Remove the struct vhost_notifier with struct kiocb.
>> 	Let vhost-net backend to alloc/free the kiocb and transfer them
>> 	via sendmsg/recvmsg.
>> 	use get_user_pages_fast() and set_page_dirty_lock() when read.
>> 	Add some comments for netdev_mp_port_prep() and handle_mpassthru().
>> Comments not addressed yet in this time:
>> 	the async write logging is not satified by vhost-net
>> 	Qemu needs a sync write
>> 	a limit for locked pages from get_user_pages_fast()
>> performance:
>> 	using netperf with GSO/TSO disabled, 10G NIC, 
>> 	disabled packet split mode, with raw socket case compared to vhost.
>> 	bindwidth will be from 1.1Gbps to 1.7Gbps
>> 	CPU % from 120%-140% to 140%-160%
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