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    SubjectRe: [PATCH v5 00/14] KVM: s390: pv: implement lazy destroy for reboot
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    On 9/20/21 15:24, Claudio Imbrenda wrote:
    > Previously, when a protected VM was rebooted or when it was shut down,
    > its memory was made unprotected, and then the protected VM itself was
    > destroyed. Looping over the whole address space can take some time,
    > considering the overhead of the various Ultravisor Calls (UVCs). This
    > means that a reboot or a shutdown would take a potentially long amount
    > of time, depending on the amount of used memory.
    >
    > This patchseries implements a deferred destroy mechanism for protected
    > guests. When a protected guest is destroyed, its memory is cleared in
    > background, allowing the guest to restart or terminate significantly
    > faster than before.
    >
    > There are 2 possibilities when a protected VM is torn down:
    > * it still has an address space associated (reboot case)
    > * it does not have an address space anymore (shutdown case)
    >
    > For the reboot case, the reference count of the mm is increased, and
    > then a background thread is started to clean up. Once the thread went
    > through the whole address space, the protected VM is actually
    > destroyed.
    >
    > This means that the same address space can have memory belonging to
    > more than one protected guest, although only one will be running, the
    > others will in fact not even have any CPUs.
    >
    > The shutdown case is more controversial, and it will be dealt with in a
    > future patchseries.
    >
    > When a guest is destroyed, its memory still counts towards its memory
    > control group until it's actually freed (I tested this experimentally)


    @Christian: I'd like to have #1-3 in early so we can focus on the more
    complicated stuff.

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