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From: Daniel Henrique Barboza
Date: Wed, 31 Mar 2021 16:47:14 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

On 3/30/21 8:46 PM, David Gibson wrote:
On Tue, Mar 30, 2021 at 01:28:31AM +0200, Igor Mammedov wrote:
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:

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.


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

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)?

Uh.. not as such.  In the PAPR model, hotplugs and unplugs are mostly
guest directed, so the question doesn't really arise.

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

Possible, but probably not that likely.  The mechanism we use to
"cancel" the hotplugs is that we just fail the hypercalls that the
guest will need to call to actually complete the hotplug.  We also
fail those in some other situations, and that seems to work.

That said, I no longer think this cancelling on timeout is a good
idea, since it mismatches what happens on other platforms more than I
think we need to.

My now preferred approach is to revert the timeout changes, but
instead allow retries of unplugs to be issued.  I think that's just a
matter of resending the unplug message to the guest, while making it
otherwise a no-op on the qemu side.

I used this approach in a patch I sent back in January:

"[PATCH v2 1/1] spapr.c: always pulse guest IRQ in spapr_core_unplug_request()"


Let me know and I'll revert the timeout mechanism and re-post this one.
I guess there's still time to make this change in the 6.0.0 window, avoiding
releasing a mechanism we're not happy with.



I believe that's already covered by the existing API:

+# 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", 
       qapi_event_send_device_unplug_error(dev->id, qapi_error);

lets make everything support ACPI (just kidding).

Heh.  If nothing else, doesn't help us with existing guests.

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

Hmm.  I'm a bit dubious about issuing ACPI messages for a non ACPI
guest, but maybe that could work.

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.



Thoughs, Markus?

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