Re: [Qemu-devel] balloon driver

[Mark Williamson]

> > Worse, the guest might decide to swap out a page that's already
> > swapped in by the host, forcing it to be read in again only to be
> > immediately written out to disk by the guest :-(
>
> ...unless the guest's disk I/O is with simulated DMA or recognisable
> block-copy instruction sequences, and doesn't look at the data.  In
> that case the emulator can, in principle, keep track of where pages
> are copied around without being examined, and avoid actually swapping
> them in.
>
> Probably not worth the complexity.

Another solution, taken by the Disco VMM was to provide a special virtual disk 
device for the swap device which fakes out swapping behaviour - accesses to 
that device are always going to be swaps, so you can guarantee to do the 
right thing without having to trace and interpret behaviour.

That said, I like your idea of not committing IOs for DMA-ed pages than have 
not been inspected, but one would have to be careful to make sure that true 
page-size DMA writes of data do still hit the disk in a timely fashion.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!