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
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