Re: [Qemu-devel] [PATCH v3 0/4] Introduce the microvm machine type

[Michael S. Tsirkin]

On Thu, Jul 25, 2019 at 05:35:01PM +0200, Paolo Bonzini wrote:
> On 25/07/19 16:46, Michael S. Tsirkin wrote:
> > Actually, I think I have a better idea.
> > At the moment we just get an exit on these reads and return all-ones.
> > Yes, in theory there could be a UR bit set in a bunch of
> > registers but in practice no one cares about these,
> > and I don't think we implement them.
> > So how about mapping a single page, read-only, and filling it
> > with all-ones?
> 
> Yes, that's nice indeed. :)  But it does have some cost, in terms of
> either number of VMAs or QEMU RSS since the MMCONFIG area is large.
> 
> What breaks if we return all zeroes?  Zero is not a valid vendor ID.
> 
> Paolo

I think I know what you are thinking of doing:
map /dev/zero so we get a single VMA but all mapped to
a single zero pte?

We could start with that, at least as an experiment.
Further:

- we can limit the amount of fragmentation and simply
  unmap everything if we exceed a specific limit:
  with more than X devices it's no longer a lightweight
  VM anyway :)

- we can implement /dev/ones. in fact, we can implement
  /dev/byteXX for each possible value, the cost will
  be only 1M on a 4k page system.
  it might come in handy for e.g. free page hinting:
  at the moment if guest memory is poisoned
  we can not unmap it, with this trick we can
  map it to /dev/byteXX.

Note that the kvm memory array is still fragmented.
Again, we can fallback on disabling the optimization
if there are too many devices.


-- 
MST