On Fri, 2010-04-02 at 15:25 +0800, xiaohui....@intel.com wrote: > 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.
What is the advantage of this approach compared to PCI-passthrough of the host NIC to the guest? Does this require pinning of the entire guest memory? Or only the send/receive buffers? Thanks Sridhar > > 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? > > 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. > > 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(). > > > 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% > -- > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
