Re: [libvirt PATCH v3 5/5] qemu: enable asynchronous teardown on s390x hosts by default

On Wed, Jul 05, 2023 at 02:46:03PM +0200, Claudio Imbrenda wrote: > On Wed, 5 Jul 2023 13:26:32 +0100 > Daniel P. Berrangé <berrange(a)redhat.com&gt; wrote: > > [...] > >>>> I rather think mgmt apps need to explicitly opt-in to async teardown, >>>> so they're aware that they need to take account of delayed RAM >>>> availablity in their accounting / guest placement logic. >>> >>> what would you think about enabling it by default only for guests that >>> are capable to run in Secure Execution mode? >> >> IIUC, that's basically /all/ guests if running on new enough hardware >> with prot_virt=1 enabled on the host OS, so will still present challenges >> to mgmt apps needing to be aware of this behaviour AFAICS. > > I think there is some fencing still? I don't think it's automatic IIUC, the following sequence is possible 1. Start QEMU with -m 500G -> QEMU spawns async teardown helper process 2. Stop QEMU -> Async teardown helper process remains running while kernel releases RAM 3. Start QEMU with -m 500G -> Fails with ENOMEM ...time passes... 4. Async teardown helper finally terminates -> The full original 500G is only now released for use Basically if you can't do while true do virsh start $guest virsh stop $guest done then it is a change in libvirt API semantics, as so will require explicit opt-in from the mgmt app to use this feature.

With regards, Daniel