Re: Ballooning on TPS!=HPS hosts

CC'ing virtualization list.

On (Thu) 31 Mar 2016 [19:00:24], Dr. David Alan Gilbert wrote:

Hi,
  I was reading the balloon code and am confused as to how/if ballooning
works on hosts where the host page size is larger than the
target page size.

static void balloon_page(void *addr, int deflate)
{
#if defined(__linux__)
    if (!qemu_balloon_is_inhibited() && (!kvm_enabled() ||
                                         kvm_has_sync_mmu())) {
        qemu_madvise(addr, TARGET_PAGE_SIZE,
                deflate ? QEMU_MADV_WILLNEED : QEMU_MADV_DONTNEED);
    }
#endif
}

The virtio-balloon code only does stuff through ballon_page,
and an madvise DONTNEED should fail if you try and do it on
a size smaller than the host page size.  So does ballooning work on
Power/ARM?

Am I misunderstanding this?

I think you're right.  Guess no one's tested this in such scenarios
yet.

Of course looking at the above we won't actually generate an error since
we don't check the return of qemu_madvise.

... at least we can deflate the balloon in case the madvise fails, so
the guest can use the pages it's given us.

We have three sizes:
    a) host page size
    b) target page size
    c) VIRTIO_BALLOON_PFN_SHIFT

 c == 12 (4k) for everyone
 

    1) I think the virtio-balloon code needs to coallesce adjecent requests
      and call balloon_page on whole chunks at once passing a length.
    2) why does balloon_page use TARGET_PAGE_SIZE, ignoring anything else
       shouldn't it be 1 << VIRTIO_BALLOON_PFN_SHIFT ?
    3) I'm guessing the guest kernel doesn't know the host page size, so
       how can it know what size chunks of balloon to work in?

Thanks,

		Amit