From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id CA9DF9863B6 for ; Tue, 13 Jul 2021 12:29:08 +0000 (UTC) From: Cornelia Huck In-Reply-To: <52f45a4e-16fb-f8c9-00a4-0a7947dcb1c5@redhat.com> References: <20210706043334.36359-1-jasowang@redhat.com> <20210706043334.36359-3-jasowang@redhat.com> <52e1347c-9363-8ff8-e36c-2681b42e4c37@redhat.com> <20210710163326-mutt-send-email-mst@kernel.org> <778ffa63-c0ca-2faa-32da-3c5e71dd1dfe@redhat.com> <3ce9775c-64fa-1c9b-6627-55f3b18ac987@redhat.com> <875yxeeicr.fsf@redhat.com> <8735sie9g1.fsf@redhat.com> <52f45a4e-16fb-f8c9-00a4-0a7947dcb1c5@redhat.com> Date: Tue, 13 Jul 2021 14:28:55 +0200 Message-ID: <87y2aacs8o.fsf@redhat.com> MIME-Version: 1.0 Subject: Re: [virtio-comment] [PATCH V2 2/2] virtio: introduce STOP status bit Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable To: Jason Wang , Stefan Hajnoczi Cc: "Michael S. Tsirkin" , Eugenio Perez Martin , "Dr. David Alan Gilbert" , virtio-comment@lists.oasis-open.org, Virtio-Dev , Max Gurtovoy , Oren Duer , Shahaf Shuler , Parav Pandit , Bodong Wang , Alexander Mikheev , Halil Pasic List-ID: On Tue, Jul 13 2021, Jason Wang wrote: > =E5=9C=A8 2021/7/13 =E4=B8=8B=E5=8D=887:31, Cornelia Huck =E5=86=99=E9=81= =93: >> On Tue, Jul 13 2021, Jason Wang wrote: >> >>> =E5=9C=A8 2021/7/13 =E4=B8=8B=E5=8D=884:19, Cornelia Huck =E5=86=99=E9= =81=93: >>>> On Tue, Jul 13 2021, Jason Wang wrote: >>>> >>>>> =E5=9C=A8 2021/7/12 =E4=B8=8B=E5=8D=885:57, Stefan Hajnoczi =E5=86=99= =E9=81=93: >>>>>> When migrating a guest with many VIRTIO devices a busy waiting appro= ach >>>>>> extends downtime if implemented sequentially (stopping one device at= a >>>>>> time). >>>>> Well. You need some kinds of waiting for sure, the device/DMA needs >>>>> sometime to be stopped. The downtime is determined by a specific virt= io >>>>> implementation which is hard to be restricted at the spec level. We c= an >>>>> clarify that the device must set the STOP bit in e.g 100ms. >>>> I don't think we can introduce arbitrary upper bounds here. At most, w= e >>>> can say that the device SHOULD try to set the STOP bit as early as >>>> possible (and make use of the mechanism to expose in-flight buffers.) >>> >>> Yes, that's my understanding. >>> >>> >>>> If we want to avoid polling for the STOP bit, we need some kind of >>>> notification mechanism, I guess. For ccw, I'd just use a channel >>>> command to stop the device; completion of that channel program would >>>> indicate that the device is done with the stop procedure. >>> >>> A question, is interrupt used for such notification, or the VMM can >>> choose to poll for the completion? >> You can poll for the subchannel to become status pending. >> >>> >>>> Not sure how >>>> well that translates to other transports. >>> >>> Actually, it's not necessarily a busy polling. VMM can schedule other >>> process in and recheck the bit periodically. >>> >>> Or as you mentioned before, we can use some kind of interrupt but it >>> would be more complicated than the simple status bit. It's better to >>> introduce the interrupt only if the status bit doesn't fit. >> At least for ccw, waiting for the status bit to be set also involves an >> interrupt or polling (we use another channel program to retrieve the >> status.) A dedicated channel command would actually be better, as the >> interrupt/status pending would already inform us of success. > > > So it looks to me it doesn't conflict with this design: the device must= =20 > wait for the device to be stopped to signal the success of the ccw comman= d? Yes, the difference is mainly how that information can be extracted. This publicly archived list offers a means to provide input to the=0D OASIS Virtual I/O Device (VIRTIO) TC.=0D =0D In order to verify user consent to the Feedback License terms and=0D to minimize spam in the list archive, subscription is required=0D before posting.=0D =0D Subscribe: virtio-comment-subscribe@lists.oasis-open.org=0D Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org=0D List help: virtio-comment-help@lists.oasis-open.org=0D List archive: https://lists.oasis-open.org/archives/virtio-comment/=0D Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf= =0D List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lis= ts=0D Committee: https://www.oasis-open.org/committees/virtio/=0D Join OASIS: https://www.oasis-open.org/join/