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From: Michael S. Tsirkin on 25 Apr 2010 08:20
On Sun, Apr 25, 2010 at 05:20:06PM +0800, xiaohui.xin(a)intel.com wrote:
> We provide an zero-copy method which driver side may get external
> buffers to DMA. Here external means driver don't use kernel space
> to allocate skb buffers. Currently the external buffer can be from
> guest virtio-net driver.
>
> 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.
>
> patch 01-12: net core changes.
> patch 13-17: new device as interface to mantpulate external buffers.
> patch 18: for vhost-net.
>
> 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?

Do you still do this?

> 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. 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.

I commented on this in the past. Do you still want comments?

> 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
> To support GRO
> 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().
>
> what we have done in v3:
> the async write logging is rewritten
> a drafted synchronous write function for qemu live migration
> a limit for locked pages from get_user_pages_fast() to prevent Dos
> by using RLIMIT_MEMLOCK
>
>
> what we have done in v4:
> add iocb completion callback from vhost-net to queue iocb in mp device
> replace vq->receiver by mp_sock_data_ready()
> remove stuff in mp device which access structures from vhost-net
> modify skb_reserve() to ignore host NIC driver reserved space
> rebase to the latest vhost tree
> split large patches into small pieces, especially for net core part.
>
>
> 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%

That's nice. The thing to do is probably to enable GSO/TSO
and see what we get this way. Also, mergeable buffer support
was recently posted and I hope to merge it for 2.6.35.
You might want to take a look.

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From: Xin, Xiaohui on 28 Apr 2010 21:40
> > 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.
> >
> >Isn't it much easier to map the RX ring of the network device into the
> >guest's address space, have DMA map calls translate guest addresses to
> >physical/DMA addresses as well as do all of this crazy page pinning
> >stuff, and provide the translations and protections via the IOMMU?

>This means we need guest know how the specific network device works.
>So we won't be able to, for example, move guest between different hosts.
>There are other problems: many physical systems do not have an iommu,
>some guest OS-es do not support DMA map calls, doing VM exit
>on each DMA map call might turn out to be very slow. And so on.

This solution is what now we can think of to implement zero-copy. Some
modifications are made to net core to try to avoid network device driver
changes. The major change is to __alloc_skb(), in which we added a dev
parameter to indicate whether the device will DMA to/from guest/user buffer
which is pointed by host skb->data. We also modify skb_release_data() and
skb_reserve(). We made it now works with ixgbe driver with PS mode disabled,
and got some performance data with it.

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%

We are now trying to get decent performance data with advanced features.
Do you have any other concerns with this solution?

>> What's being proposed here looks a bit over-engineered.

>This is an attempt to reduce overhead for virtio (paravirtualization).
>'Don't use PV' is kind of an alternative, but I do not
>think it's a simpler one.

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From: Xin, Xiaohui on 8 May 2010 04:00
Michael,
Sorry, somehow I missed this mail. :-(

>> 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?

>Do you still do this?

Currently, we still use the first way. But we now ignore the room which
host skb_reserve() required when device is doing zero-copy. Now we don't
taint guest virtio-net driver with a new method by this way.

>> 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. 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.

>I commented on this in the past. Do you still want comments?

Now we continue with the first way and try to push it. But any comments about the two methods are still welcome.

>That's nice. The thing to do is probably to enable GSO/TSO
>and see what we get this way. Also, mergeable buffer support
>was recently posted and I hope to merge it for 2.6.35.
>You might want to take a look.

I'm looking at the mergeable buffer. I think GSO/GRO support with zero-copy also needs it.
Currently, GSO/TSO is still not supported by vhost-net?
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From: Michael S. Tsirkin on 9 May 2010 05:40
On Sat, May 08, 2010 at 03:55:48PM +0800, Xin, Xiaohui wrote:
> Michael,
> Sorry, somehow I missed this mail. :-(
>
> >> 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?
>
> >Do you still do this?
>
> Currently, we still use the first way. But we now ignore the room which
> host skb_reserve() required when device is doing zero-copy. Now we don't
> taint guest virtio-net driver with a new method by this way.
>
> >> 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. 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.
>
> >I commented on this in the past. Do you still want comments?
>
> Now we continue with the first way and try to push it. But any comments about the two methods are still welcome.
>
> >That's nice. The thing to do is probably to enable GSO/TSO
> >and see what we get this way. Also, mergeable buffer support
> >was recently posted and I hope to merge it for 2.6.35.
> >You might want to take a look.
>
> I'm looking at the mergeable buffer. I think GSO/GRO support with zero-copy also needs it.
> Currently, GSO/TSO is still not supported by vhost-net?

GSO/TSO are currently supported with tap and macvtap,
AF_PACKET socket backend still needs some work to
enable GSO.

> --
> MST
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