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

From: Igor Mammedov <imammedo@redhat.com>
To: Daniel Henrique Barboza <danielhb413@gmail.com>
Cc: qemu-devel@nongnu.org, groug@kaod.org, qemu-ppc@nongnu.org,
	armbru@redhat.com, David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [PATCH 0/4] DEVICE_NOT_DELETED/DEVICE_UNPLUG_ERROR QAPI events
Date: Tue, 30 Mar 2021 01:28:31 +0200	[thread overview]
Message-ID: <20210330012831.2ce0514c@redhat.com> (raw)
In-Reply-To: <ba20de28-d65b-6da4-5bff-92b637cf7a56@gmail.com>

On Wed, 24 Mar 2021 16:09:59 -0300
Daniel Henrique Barboza <danielhb413@gmail.com> wrote:

> On 3/23/21 10:40 PM, David Gibson wrote:
> > 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).

Is canceling unplug on timeout documented somewhere (like some spec)?

If not it might (theoretically) confuse guest when it tries to unplug
after timeout and leave guest in some unexpected state. 

> 
> I believe that's already covered by the existing API:
> 
> 
> +# @DEVICE_UNPLUG_ERROR:
> +#
> +# Emitted when a device hot unplug error occurs.
> +#
> +# @device: device name
> +#
> +# @msg: Informative message
> 
> The 'informative message' would be the reason the event occurred. In patch
> 4/4, for the memory hotunplug refused by the guest, it is being set as:
> 
>       qapi_error = g_strdup_printf("Memory hotunplug rejected by the guest "
>                                    "for device %s", dev->id);
>       qapi_event_send_device_unplug_error(dev->id, qapi_error);
> 
> 
> 
> We could use the same DEVICE_UNPLUG_ERROR event in the CPU hotunplug timeout
> case (currently on patch 2/4) by just changing 'msg', e.g.:
> 
> 
>       qapi_error = g_strdup_printf("CPU hotunplug timeout for device %s", dev->id);
>       qapi_event_send_device_unplug_error(dev->id, qapi_error);
> 

lets make everything support ACPI (just kidding).

maybe we can reuse already existing ACPI_DEVICE_OST instead of DEVICE_UNPLUG_ERROR
which sort of does the same thing (and more) but instead of strings uses status codes
defined by spec.

Idea similar to DEVICE_UNPLUG_ERROR was considered back then, but instead of QEMU being
a poor translator of status codes to non machine-readable strings we went with
exposing well documented status codes to user. This way user can implement
specific reactions to particular errors just looking at JSON + spec.

> Thanks,
> 
> DHB
> 
> 
> > 
> > Thoughs, Markus?
> >   
> 