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> 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
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