Re: [PATCH v1 00/11] KVM: s390: pv: implement lazy destroy

[Claudio Imbrenda]

On Tue, 18 May 2021 17:05:37 +0200
Cornelia Huck <cohuck@xxxxxxxxxx> wrote:

> On Mon, 17 May 2021 22:07:47 +0200
> Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx> 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.
> > 
> > For the shutdown case, a list of pages to be destroyed is formed
> > when the mm is torn down. Instead of just unmapping the pages when
> > the address space is being torn down, they are also set aside.
> > Later when KVM cleans up the VM, a thread is started to clean up
> > the pages from the list.  
> 
> Just to make sure, 'clean up' includes doing uv calls?

yes

> > 
> > 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.  
> 
> Are those set-aside-but-not-yet-cleaned-up pages still possibly
> accessible in any way? I would assume that they only belong to the

in case of reboot: yes, they are still in the address space of the
guest, and can be swapped if needed

> 'zombie' guests, and any new or rebooted guest is a new entity that
> needs to get new pages?

the rebooted guest (normal or secure) will re-use the same pages of the
old guest (before or after cleanup, which is the reason of patches 3
and 4)

the KVM guest is not affected in case of reboot, so the userspace
address space is not touched.

> Can too many not-yet-cleaned-up pages lead to a (temporary) memory
> exhaustion?

in case of reboot, not much; the pages were in use are still in use
after the reboot, and they can be swapped.

in case of a shutdown, yes, because the pages are really taken aside
and cleared/destroyed in background. they cannot be swapped. they are
freed immediately as they are processed, to try to mitigate memory
exhaustion scenarios.

in the end, this patchseries is a tradeoff between speed and memory
consumption. the memory needs to be cleared up at some point, and that
requires time.

in cases where this might be an issue, I introduced a new KVM flag to
disable lazy destroy (patch 10)