Re: [PATCH 0/4] DEVICE_NOT_DELETED/DEVICE_UNPLUG_ERROR QAPI events

From: David Gibson <david@gibson.dropbear.id.au>
To: Daniel Henrique Barboza <danielhb413@gmail.com>
Cc: armbru@redhat.com, qemu-ppc@nongnu.org, qemu-devel@nongnu.org,
	groug@kaod.org
Subject: Re: [PATCH 0/4] DEVICE_NOT_DELETED/DEVICE_UNPLUG_ERROR QAPI events
Date: Wed, 24 Mar 2021 12:40:34 +1100	[thread overview]
Message-ID: <YFqYkuBSD3xPgLVi@yekko.fritz.box> (raw)
In-Reply-To: <b8d96ab9-61e5-b941-6405-5beff5fe0df5@gmail.com>

[-- Attachment #1: Type: text/plain, Size: 2629 bytes --]

On Tue, Mar 23, 2021 at 02:10:22PM -0300, Daniel Henrique Barboza wrote:
> 
> 
> On 3/22/21 10:12 PM, David Gibson wrote:
> > On Fri, Mar 12, 2021 at 05:07:36PM -0300, Daniel Henrique Barboza wrote:
> > > Hi,
> > > 
> > > This series adds 2 new QAPI events, DEVICE_NOT_DELETED and
> > > DEVICE_UNPLUG_ERROR. They were (and are still being) discussed in [1].
> > > 
> > > Patches 1 and 3 are independent of the ppc patches and can be applied
> > > separately. Patches 2 and 4 are based on David's ppc-for-6.0 branch and
> > > are dependent on the QAPI patches.
> > 
> > Implementation looks fine, but I think there's a bit more to discuss
> > before we can apply.
> > 
> > I think it would make sense to re-order this and put UNPLUG_ERROR
> > first.  Its semantics are clearer, and I think there's a stronger case
> > for it.
> 
> Alright
> 
> > 
> > I'm a bit less sold on DEVICE_NOT_DELETED, after consideration.  Does
> > it really tell the user/management anything useful beyond what
> > receiving neither a DEVICE_DELETED nor a DEVICE_UNPLUG_ERROR does?
> 
> 
> It informs that the hotunplug operation exceed the timeout that QEMU
> internals considers adequate, but QEMU can't assert that it was caused
> by an error or an unexpected delay. The end result is that the device
> is not going to be deleted from QMP, so DEVICE_NOT_DELETED.

Is it, though?  I mean, it is with this implementation for papr:
because we clear the unplug_requested flag, even if the guest later
tries to complete the unplug, it will fail.

But if I understand what Markus was saying correctly, that might not
be possible for all hotplug systems.  I believe Markus was suggesting
that DEVICE_NOT_DELETED could just mean that we haven't deleted the
device yet, but it could still happen later.

And in that case, I'm not yet sold on the value of a message that
essentially just means "Ayup, still dunno what's happening, sorry".

> Perhaps we should just be straightforward and create a DEVICE_UNPLUG_TIMEOUT
> event.

Hm... what if we added a "reason" field to UNPLUG_ERROR.  That could
be "guest rejected hotplug", or something more specific, in the rare
case that the guest has a way of signalling something more specific,
or "timeout" - but the later *only* to be sent in cases where on the
timeout we're able to block any later completion of the unplug (as we
can on papr).

Thoughs, Markus?

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]