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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
