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[Qemu-devel] Ballooning on TPS!=HPS hosts

From: Dr. David Alan Gilbert
Subject: [Qemu-devel] Ballooning on TPS!=HPS hosts
Date: Thu, 31 Mar 2016 19:00:24 +0100
User-agent: Mutt/1.5.24 (2015-08-30)

  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);

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

Am I misunderstanding this?

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

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

 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?


Dr. David Alan Gilbert / address@hidden / Manchester, UK

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