From: Claudio Imbrenda <imbrenda@linux.ibm.com>
To: Cornelia Huck <cohuck@redhat.com>
Cc: kvm@vger.kernel.org, borntraeger@de.ibm.com,
frankja@linux.ibm.com, thuth@redhat.com, pasic@linux.ibm.com,
david@redhat.com, linux-s390@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 00/11] KVM: s390: pv: implement lazy destroy
Date: Tue, 18 May 2021 17:36:24 +0200 [thread overview]
Message-ID: <20210518173624.13d043e3@ibm-vm> (raw)
In-Reply-To: <20210518170537.58b32ffe.cohuck@redhat.com>
On Tue, 18 May 2021 17:05:37 +0200
Cornelia Huck <cohuck@redhat.com> wrote:
> On Mon, 17 May 2021 22:07:47 +0200
> Claudio Imbrenda <imbrenda@linux.ibm.com> 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)
next prev parent reply other threads:[~2021-05-18 15:36 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-17 20:07 [PATCH v1 00/11] KVM: s390: pv: implement lazy destroy Claudio Imbrenda
2021-05-17 20:07 ` [PATCH v1 01/11] KVM: s390: pv: leak the ASCE page when destroy fails Claudio Imbrenda
2021-05-18 10:26 ` Janosch Frank
2021-05-18 10:40 ` Claudio Imbrenda
2021-05-18 12:00 ` Janosch Frank
2021-05-17 20:07 ` [PATCH v1 02/11] KVM: s390: pv: properly handle page flags for protected guests Claudio Imbrenda
2021-05-17 20:07 ` [PATCH v1 03/11] KVM: s390: pv: handle secure storage violations " Claudio Imbrenda
2021-05-17 20:07 ` [PATCH v1 04/11] KVM: s390: pv: handle secure storage exceptions for normal guests Claudio Imbrenda
2021-05-17 20:07 ` [PATCH v1 05/11] KVM: s390: pv: refactor s390_reset_acc Claudio Imbrenda
2021-05-26 12:11 ` Janosch Frank
2021-05-17 20:07 ` [PATCH v1 06/11] KVM: s390: pv: usage counter instead of flag Claudio Imbrenda
2021-05-27 9:29 ` Janosch Frank
2021-05-17 20:07 ` [PATCH v1 07/11] KVM: s390: pv: add export before import Claudio Imbrenda
2021-05-26 11:56 ` Janosch Frank
2021-05-17 20:07 ` [PATCH v1 08/11] KVM: s390: pv: lazy destroy for reboot Claudio Imbrenda
2021-05-27 9:43 ` Janosch Frank
2021-05-17 20:07 ` [PATCH v1 09/11] KVM: s390: pv: extend lazy destroy to handle shutdown Claudio Imbrenda
2021-05-17 20:07 ` [PATCH v1 10/11] KVM: s390: pv: module parameter to fence lazy destroy Claudio Imbrenda
2021-05-27 10:35 ` Janosch Frank
2021-05-17 20:07 ` [PATCH v1 11/11] KVM: s390: pv: add support for UV feature bits Claudio Imbrenda
2021-05-18 15:05 ` [PATCH v1 00/11] KVM: s390: pv: implement lazy destroy Cornelia Huck
2021-05-18 15:36 ` Claudio Imbrenda [this message]
2021-05-18 15:45 ` Christian Borntraeger
2021-05-18 15:52 ` Cornelia Huck
2021-05-18 16:13 ` Claudio Imbrenda
2021-05-18 16:20 ` Christian Borntraeger
2021-05-18 16:34 ` Claudio Imbrenda
2021-05-18 16:35 ` Christian Borntraeger
2021-05-18 16:04 ` Cornelia Huck
2021-05-18 16:19 ` Claudio Imbrenda
2021-05-18 16:22 ` David Hildenbrand
2021-05-18 16:31 ` Claudio Imbrenda
2021-05-18 16:55 ` Christian Borntraeger
2021-05-18 17:00 ` Claudio Imbrenda
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