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    SubjectRe: [RFC][PATCH] virtio balloon: kill tell-host-first logic
    On Tue, 2011-04-12 at 11:13 +0530, Amit Shah wrote:
    > Sure, the only contention was on the commit message, where you stated
    > modern qemus set this... qemu doesn't, and it should. Care to do a
    > patch for that?

    If Rusty hasn't pushed the commit out anywhere, we can still amend the
    commit. Otherwise, we're in a _bit_ of a pickle since you can't patch
    git logs. :)

    Whatever is easiest for Rusty works for me.

    How about this for a replacement log?


    The virtio balloon driver has a VIRTIO_BALLOON_F_MUST_TELL_HOST
    feature bit. Whenever the bit is set, we must always tell the
    host before we free pages back to the allocator. Without this
    feature, we might free a page (and have another user touch it)
    while the hypervisor is unprepared for it.

    But, if the bit is _not_ set, we are under no obligation to
    reverse the order; we're under no obligation to do _anything_.
    That's the state of affairs in current qemu:


    This patch makes the "tell host first" logic the only case. This
    should make everybody happy, and reduce the amount of untested or
    untestable code in the kernel.

    This _also_ means that we don't have to preserve a pfn list
    after the pages are freed, which should let us get rid of some
    temporary storage (vb->pfns) eventually.

    Signed-off-by: Dave Hansen <>

    -- Dave

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