linux-kernel - Re: [PATCH v14 1/6] KVM: s390: pv: asynchronous destroy for reboot

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Date:   Mon, 10 Oct 2022 13:38:13 +0200
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     borntraeger@...ibm.com, thuth@...hat.com, david@...hat.com,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        scgl@...ux.ibm.com, seiden@...ux.ibm.com, nrb@...ux.ibm.com
Subject: Re: [PATCH v14 1/6] KVM: s390: pv: asynchronous destroy for reboot

On 9/30/22 16:01, Claudio Imbrenda wrote:
> Until now, destroying a protected guest was an entirely synchronous
> operation that could potentially take a very long time, depending on
> the size of the guest, due to the time needed to clean up the address
> space from protected pages.
> 
> This patch implements an asynchronous destroy mechanism, that allows a
> protected guest to reboot significantly faster than previously.
> 
> This is achieved by clearing the pages of the old guest in background.
> In case of reboot, the new guest will be able to run in the same
> address space almost immediately.
> 
> The old protected guest is then only destroyed when all of its memory
> has been destroyed or otherwise made non protected.
> 
> Two new PV commands are added for the KVM_S390_PV_COMMAND ioctl:
> 
> KVM_PV_ASYNC_CLEANUP_PREPARE: set aside the current protected VM for
> later asynchronous teardown. The current KVM VM will then continue
> immediately as non-protected. If a protected VM had already been
> set aside for asynchronous teardown, but without starting the teardown
> process, this call will fail. There can be at most one VM set aside at
> any time. Once it is set aside, the protected VM only exists in the
> context of the Ultravisor, it is not associated with the KVM VM
> anymore. Its protected CPUs have already been destroyed, but not its
> memory. This command can be issued again immediately after starting
> KVM_PV_ASYNC_CLEANUP_PERFORM, without having to wait for completion.
> 
> KVM_PV_ASYNC_CLEANUP_PERFORM: tears down the protected VM previously
> set aside using KVM_PV_ASYNC_CLEANUP_PREPARE. Ideally the
> KVM_PV_ASYNC_CLEANUP_PERFORM PV command should be issued by userspace
> from a separate thread. If a fatal signal is received (or if the
> process terminates naturally), the command will terminate immediately
> without completing. All protected VMs whose teardown was interrupted
> will be put in the need_cleanup list. The rest of the normal KVM
> teardown process will take care of properly cleaning up all remaining
> protected VMs, including the ones on the need_cleanup list.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> Reviewed-by: Nico Boehr <nrb@...ux.ibm.com>

That was quite a read.

With Steffen's nits fixed:
Reviewed-by: Janosch Frank <frankja@...ux.ibm.com>