On 04/25/2011 08:49 PM, David Ahern wrote:
> > There are several copies. > > qemu's virtio-net implementation incurs a copy on tx and on rx when > calling the kernel; in addition there is also an internal copy: > > /* copy in packet. ugh */ > len = iov_from_buf(sg, elem.in_num, > buf + offset, size - offset); > > In principle vhost-net can avoid the tx copy, but I think now we have 1 > copy on rx and tx each. So there is a copy internal to qemu, then from qemu to the host tap device and then tap device to a physical NIC if the packet is leaving the host?
There is no internal copy on tx, just rx. So: virtio-net: 1 internal rx, 1 kernel/user rx, 1 kernel/user tx vhost-net: 1 internal rx, 1 internal tx
Is that what the zero-copy patch set is attempting - bypassing the transmit copy to the macvtap device?
Yes.
> > If a host interface is dedicated to backing a vhost-net interface (say > if you have an SR/IOV card) then you can in principle avoid the rx copy > as well. > > An alternative to avoiding the copies is to use a dma engine, like I > mentioned. > How does the DMA engine differ from the zero-copy patch set?
The DMA engine does not avoid the copy, it merely uses a device other than the cpu to perform it. It offloads the cpu but still loads the interconnect. True zero-copy avoids both the cpu load and the interconnect load.
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