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Subject: Re: [PATCH v5 00/14] KVM: s390: pv: implement lazy destroy for reboot
To:     Janosch Frank <frankja@linux.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>, kvm@vger.kernel.org
Cc:     cohuck@redhat.com, thuth@redhat.com, pasic@linux.ibm.com,
        david@redhat.com, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, Ulrich.Weigand@de.ibm.com
References: <20210920132502.36111-1-imbrenda@linux.ibm.com>
 <fcfd5d04-1a08-f91e-7bc2-8878c6dcd1eb@linux.ibm.com>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <566654e2-92fd-4e91-325e-ced6a89b7a0e@de.ibm.com>
Date:   Tue, 5 Oct 2021 16:15:20 +0200
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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.