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

[Daniel P. Berrangé]

On Tue, Jul 11, 2023 at 04:22:12PM +0200, Claudio Imbrenda wrote:
> On Tue, 11 Jul 2023 14:57:45 +0100
> Daniel P. Berrangé <berrange at redhat.com> wrote:
> 
> > On Tue, Jul 11, 2023 at 03:48:25PM +0200, Claudio Imbrenda wrote:
> > > On Tue, 11 Jul 2023 09:17:00 +0100
> > > Daniel P. Berrangé <berrange at redhat.com> wrote:
> > > 
> > > [...]
> > >   
> > > > > We could add additional time depending on the guest memory size BUT with
> > > > > Secure Execution the timeout would need to be increased by factors (two
> > > > > digits). Also for libvirt it is not possible to detect if the guest is in
> > > > > Secure Execution mode.    
> > > > 
> > > > What component is causing this 2 orders of magnitude delay in shutting  
> > > 
> > > Secure Execution (protected VMs)  
> > 
> > So its the hardware that imposes the penalty, rather than something
> > the kenrel is doing ?
> > 
> > Can anything else mitigate this ?  eg does using huge pages make it
> > faster than normal pages ?
> 
> unfortunately huge pages cannot be used for Secure Execution, it's a
> hardware limitation.
> 
> > 
> > > > down a guest ? If the host can't tell if Secure Execution mode is
> > > > enabled or not, why would any code path be different & slower ?  
> > > 
> > > The host kernel (and QEMU) know if a specific VM is running in
> > > secure mode, but there is no meaningful way for this information to be
> > > communicated outwards (e.g. to libvirt)  
> > 
> > Can we expose this in one of the QMP commands, or a new one ? It feels
> > like a mgmt app is going to want to know if a guest is running in secure
> > mode or not, so it can know if this shutdown penalty is going to be
> > present.
> 
> I guess it would be possible (no idea how easy/clean it would be). the
> issue would be that when the guest is running, it's too late to enable
> asynchronous teardown.

I think just need to document that async teardown is highly recommended
regardless.  The ability to query  secure virt, is more about helping
the application know whether async teardown will be fast or very very
slow.

> also notice that the same guest can jump in and out of secure mode
> without needing to shut down (a reboot is enough)

Yep, though I imagine that's going to be fairly unlikely in practice.

> > > During teardown, the host kernel will need to do some time-consuming
> > > extra cleanup for each page that belonged to a secure guest.
> > >   
> > > >   
> > > > > I also assume that timeouts of +1h are not acceptable. Wouldn't a long
> > > > > timeout cause other trouble like stalling "virsh list" run in parallel?    
> > > > 
> > > > Well a 1 hour timeout is pretty insane, even with the async teardown  
> > > 
> > > I think we all agree, and that's why asynchronous teardown was
> > > implemented
> > >   
> > > > that's terrible as RAM is unable to be used for any new guest for
> > > > an incredibly long time.  
> > > 
> > > I'm not sure what you mean here. RAM is not kept aside until the
> > > teardown is complete; cleared pages are returned to the free pool
> > > immediately as they are cleared. i.e. when the cleanup is halfway
> > > through, half of the memory will have been freed.  
> > 
> > Yes, it is incrementally released, but in practice most hypervisors are
> > memory constrained. So if you stop a 2 TB guest, and want to then boot it
> > again, unless you have a couple of free TB of RAM hanging around, you're
> > going to need to wait for most all of the orignial RAM to be reclaimed.
> 
> if it's a secure guest, it will take time to actually use the memory
> anyway. it's a similar issue to the teardown, but in reverse.

Unless the guest is started with memory preallocation on the QEMU side
which would make QEMU touch every page to fault it into RAM.

With regards,
Daniel
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