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([2804:431:c7c6:e000:6f43:93dd:11a0:93a1]) by smtp.gmail.com with ESMTPSA id h7sm2170109qkk.41.2021.03.31.12.47.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 31 Mar 2021 12:47:19 -0700 (PDT) Subject: Re: [PATCH 0/4] DEVICE_NOT_DELETED/DEVICE_UNPLUG_ERROR QAPI events To: David Gibson , Igor Mammedov References: <20210312200740.815014-1-danielhb413@gmail.com> <20210330012831.2ce0514c@redhat.com> From: Daniel Henrique Barboza Message-ID: 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 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2607:f8b0:4864:20::832; envelope-from=danielhb413@gmail.com; helo=mail-qt1-x832.google.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_ENVFROM_END_DIGIT=0.25, FREEMAIL_FROM=0.001, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: groug@kaod.org, qemu-ppc@nongnu.org, armbru@redhat.com, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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 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)? > > 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()" https://lists.gnu.org/archive/html/qemu-devel/2021-01/msg04399.html 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. Thanks, DHB > >>> 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). > > Heh. If nothing else, doesn't help us with existing guests. > >> 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. > > 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. >> >>> Thanks, >>> >>> DHB >>> >>> >>>> >>>> Thoughs, Markus? >>>> >>> >> >