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

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Date:   Tue, 5 Oct 2021 16:15:20 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     cohuck@...hat.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



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.