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

[Daniel P. Berrangé]

On Wed, Jul 05, 2023 at 03:29:39PM +0200, Claudio Imbrenda wrote:
> On Wed, 5 Jul 2023 14:08:27 +0100
> Daniel P. Berrangé <berrange at redhat.com> wrote:
> 
> > 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 at 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  
> 
> not running, the process terminates immediately as soon as QEMU
> terminates. the termination takes some time, because of the memory
> cleanup.
> 
> >          kernel releases RAM
> >   3. Start QEMU with -m 500G
> >       -> Fails with ENOMEM  
> 
> why though? the new VM will not manage to instantly use all of the
> memory
> 
> >   ...time passes...
> >   4. Async teardown helper finally terminates
> >       -> The full original 500G is only now released for use  
> 
> memory starts to get freed as soon as the helper process terminates
> (which is as immediately as possible after QEMU terminates
> 
> so unless you have a guest that will allocate and use all of its memory
> immediately as fast as possible at boot, this won't be a concern.

When using huge pages, QEMU should be fully allocating memory
immediately, regardless of whether the guest OS touches all RAM.


With regards,
Daniel
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