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

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Message-ID: <fcfd5d04-1a08-f91e-7bc2-8878c6dcd1eb@linux.ibm.com>
Date:   Tue, 5 Oct 2021 15:26:23 +0200
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     cohuck@...hat.com, borntraeger@...ibm.com, thuth@...hat.com,
        pasic@...ux.ibm.com, david@...hat.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ulrich.Weigand@...ibm.com
Subject: Re: [PATCH v5 00/14] KVM: s390: pv: implement lazy destroy for reboot

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.