On Tue, 18 May 2021 17:05:37 +0200 Cornelia Huck <cohuck@xxxxxxxxxx> wrote: > On Mon, 17 May 2021 22:07:47 +0200 > Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx> 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)
