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 _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization