Re: [PATCH v5 00/14] KVM: s390: pv: implement lazy destroy for reboot

[Christian Borntraeger]

Am 05.10.21 um 15:26 schrieb Janosch Frank:
> 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.

Yes, makes perfect sense.