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Tue, 18 May 2021 15:36:26 +0000 (GMT) Received: from ibm-vm (unknown [9.145.14.34]) by d06av21.portsmouth.uk.ibm.com (Postfix) with ESMTP id C474E5204E; Tue, 18 May 2021 15:36:25 +0000 (GMT) Date: Tue, 18 May 2021 17:36:24 +0200 From: Claudio Imbrenda To: Cornelia Huck 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 Message-ID: <20210518173624.13d043e3@ibm-vm> In-Reply-To: <20210518170537.58b32ffe.cohuck@redhat.com> References: <20210517200758.22593-1-imbrenda@linux.ibm.com> <20210518170537.58b32ffe.cohuck@redhat.com> Organization: IBM X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: SlwbSI_7HWxHxEdtH9xjyhEamKTw-7I7 X-Proofpoint-ORIG-GUID: 75XW01xijpZLMP_SZ34jtcS81s4v_6De X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.391,18.0.761 definitions=2021-05-18_07:2021-05-18,2021-05-18 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 malwarescore=0 impostorscore=0 suspectscore=0 clxscore=1015 mlxlogscore=953 phishscore=0 mlxscore=0 adultscore=0 spamscore=0 priorityscore=1501 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2104190000 definitions=main-2105180111 Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Tue, 18 May 2021 17:05:37 +0200 Cornelia Huck wrote: > On Mon, 17 May 2021 22:07:47 +0200 > 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. > > > > 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)