Re: [PATCH 4/5] kvm: qemu: Use vringfd to eliminate copies

[Avi Kivity <avi@qumranet.com>]

Anthony Liguori wrote:
>>
>> In theory vringfd will get us zero copy from guest sendfile out to 
>> external machines.  For anything else we're doing a copy anyway, so 
>> avoiding copying has no great benefit.
>>   
>
> There's nothing that prevents zero-copy to be implemented for tun 
> without vringfd.  In fact, I seem to recall that your earlier patches 
> implemented zero-copy :-)
>
> I like the vringfd model and I think it's a good way to move forward.  
> My concern is that it introduces an extra syscall in the TX path.  
> Right now, we do a single write call whereas with vringfd we need to 
> insert the TX packet into the queue, do a notify, and then wait for 
> indication that the TX has succeeded.
>
> I know we'll win with TSO but we don't need vringfd for TSO.  The 
> jury's still out IMHO as to whether we should do vringfd or just try 
> to merge TSO tun patches.

tun+tso still doesn't give you zerocopy (unless you change it to use 
aio, which re-introduces the syscall).

btw, the two vringfd syscalls are amortized over a potentially large 
number of packets, whereas the single tun syscall is per-packet.


(note: we can get rid of the two syscalls as well by having each side 
opportunistically pick up ring entries, like Xen does)

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.