From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52518) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1duC22-00058M-VL for qemu-devel@nongnu.org; Tue, 19 Sep 2017 02:29:20 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1duC1z-00071L-KR for qemu-devel@nongnu.org; Tue, 19 Sep 2017 02:29:18 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36366) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1duC1z-00070e-BI for qemu-devel@nongnu.org; Tue, 19 Sep 2017 02:29:15 -0400 Date: Tue, 19 Sep 2017 14:29:03 +0800 From: Peter Xu Message-ID: <20170919062903.GH3617@pxdev.xzpeter.org> References: <20170914185314.GA3280@work-vm> <20170915044622.GO3617@pxdev.xzpeter.org> <20170918083737.GD3617@pxdev.xzpeter.org> <20170918105516.GD2581@work-vm> <20170918112618.GF2581@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [RFC 00/15] QMP: out-of-band (OOB) execution support List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Cc: "Dr. David Alan Gilbert" , QEMU , Paolo Bonzini , "Daniel P . Berrange" , Stefan Hajnoczi , Fam Zheng , Juan Quintela , Michael Roth , Eric Blake , Laurent Vivier , Markus Armbruster On Mon, Sep 18, 2017 at 06:09:29PM +0200, Marc-Andr=C3=A9 Lureau wrote: > On Mon, Sep 18, 2017 at 1:26 PM, Dr. David Alan Gilbert > wrote: > > * Marc-Andr=C3=A9 Lureau (marcandre.lureau@gmail.com) wrote: > >> Hi > >> > >> On Mon, Sep 18, 2017 at 12:55 PM, Dr. David Alan Gilbert > >> wrote: > >> > * Marc-Andr=C3=A9 Lureau (marcandre.lureau@gmail.com) wrote: > >> >> Hi > >> >> > >> >> On Mon, Sep 18, 2017 at 10:37 AM, Peter Xu wr= ote: > >> >> > On Fri, Sep 15, 2017 at 01:14:47PM +0200, Marc-Andr=C3=A9 Lurea= u wrote: > >> >> >> Hi > >> >> >> > >> >> >> On Thu, Sep 14, 2017 at 9:46 PM, Peter Xu = wrote: > >> >> >> > On Thu, Sep 14, 2017 at 07:53:15PM +0100, Dr. David Alan Gil= bert wrote: > >> >> >> >> * Marc-Andr=C3=A9 Lureau (marcandre.lureau@gmail.com) wrote= : > >> >> >> >> > Hi > >> >> >> >> > > >> >> >> >> > On Thu, Sep 14, 2017 at 9:50 AM, Peter Xu wrote: > >> >> >> >> > > This series was born from this one: > >> >> >> >> > > > >> >> >> >> > > https://lists.gnu.org/archive/html/qemu-devel/2017-08= /msg04310.html > >> >> >> >> > > > >> >> >> >> > > The design comes from Markus, and also the whole-bunch-= of discussions > >> >> >> >> > > in previous thread. My heartful thanks to Markus, Dani= el, Dave, > >> >> >> >> > > Stefan, etc. on discussing the topic (...again!), provi= ding shiny > >> >> >> >> > > ideas and suggestions. Finally we got such a solution = that seems to > >> >> >> >> > > satisfy everyone. > >> >> >> >> > > > >> >> >> >> > > I re-started the versioning since this series is totall= y different > >> >> >> >> > > from previous one. Now it's version 1. > >> >> >> >> > > > >> >> >> >> > > In case new reviewers come along the way without readin= g previous > >> >> >> >> > > discussions, I will try to do a summary on what this is= all about. > >> >> >> >> > > > >> >> >> >> > > What is OOB execution? > >> >> >> >> > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D > >> >> >> >> > > > >> >> >> >> > > It's the shortcut of Out-Of-Band execution, its name is= given by > >> >> >> >> > > Markus. It's a way to quickly execute a QMP request. = Say, originally > >> >> >> >> > > QMP is going throw these steps: > >> >> >> >> > > > >> >> >> >> > > JSON Parser --> QMP Dispatcher --> Respond > >> >> >> >> > > /|\ (2) (3) | > >> >> >> >> > > (1) | \|/ (4) > >> >> >> >> > > +--------- main thread --------+ > >> >> >> >> > > > >> >> >> >> > > The requests are executed by the so-called QMP-dispatch= er after the > >> >> >> >> > > JSON is parsed. If OOB is on, we run the command direc= tly in the > >> >> >> >> > > parser and quickly returns. > >> >> >> >> > > >> >> >> >> > All commands should have the "id" field mandatory in this= case, else > >> >> >> >> > the client will not distinguish the replies coming from t= he last/oob > >> >> >> >> > and the previous commands. > >> >> >> >> > > >> >> >> >> > This should probably be enforced upfront by client capabi= lity checks, > >> >> >> >> > more below. > >> >> >> > > >> >> >> > Hmm yes since the oob commands are actually running in async= way, > >> >> >> > request ID should be needed here. However I'm not sure whet= her > >> >> >> > enabling the whole "request ID" thing is too big for this "t= ry to be > >> >> >> > small" oob change... And IMHO it suites better to be part of= the whole > >> >> >> > async work (no matter which implementation we'll use). > >> >> >> > > >> >> >> > How about this: we make "id" mandatory for "run-oob" request= s only. > >> >> >> > For oob commands, they will always have ID then no ordering = issue, and > >> >> >> > we can do it async; for the rest of non-oob commands, we sti= ll allow > >> >> >> > them to go without ID, and since they are not oob, they'll a= lways be > >> >> >> > done in order as well. Would this work? > >> >> >> > >> >> >> This mixed-mode is imho more complicated to deal with than hav= ing the > >> >> >> protocol enforced one way or the other, but that should work. > >> >> >> > >> >> >> > > >> >> >> >> > > >> >> >> >> > > Yeah I know in current code the parser calls dispatcher= directly > >> >> >> >> > > (please see handle_qmp_command()). However it's not tr= ue again after > >> >> >> >> > > this series (parser will has its own IO thread, and dis= patcher will > >> >> >> >> > > still be run in main thread). So this OOB does brings = something > >> >> >> >> > > different. > >> >> >> >> > > > >> >> >> >> > > There are more details on why OOB and the difference/re= lationship > >> >> >> >> > > between OOB, async QMP, block/general jobs, etc.. but I= MHO that's > >> >> >> >> > > slightly out of topic (and believe me, it's not easy fo= r me to > >> >> >> >> > > summarize that). For more information, please refers t= o [1]. > >> >> >> >> > > > >> >> >> >> > > Summary ends here. > >> >> >> >> > > > >> >> >> >> > > Some Implementation Details > >> >> >> >> > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D > >> >> >> >> > > > >> >> >> >> > > Again, I mentioned that the old QMP workflow is this: > >> >> >> >> > > > >> >> >> >> > > JSON Parser --> QMP Dispatcher --> Respond > >> >> >> >> > > /|\ (2) (3) | > >> >> >> >> > > (1) | \|/ (4) > >> >> >> >> > > +--------- main thread --------+ > >> >> >> >> > > > >> >> >> >> > > What this series does is, firstly: > >> >> >> >> > > > >> >> >> >> > > JSON Parser QMP Dispatcher --> Respond > >> >> >> >> > > /|\ | /|\ (4) | > >> >> >> >> > > | | (2) | (3) | (5) > >> >> >> >> > > (1) | +-----> | \|/ > >> >> >> >> > > +--------- main thread <-------+ > >> >> >> >> > > > >> >> >> >> > > And further: > >> >> >> >> > > > >> >> >> >> > > queue/kick > >> >> >> >> > > JSON Parser =3D=3D=3D=3D=3D=3D> QMP Dispatcher -->= Respond > >> >> >> >> > > /|\ | (3) /|\ (4) | > >> >> >> >> > > (1) | | (2) | | (5) > >> >> >> >> > > | \|/ | \|/ > >> >> >> >> > > IO thread main thread <-------+ > >> >> >> >> > > >> >> >> >> > Is the queue per monitor or per client? > >> >> >> > > >> >> >> > The queue is currently global. I think yes maybe at least we= can do it > >> >> >> > per monitor, but I am not sure whether that is urgent or can= be > >> >> >> > postponed. After all now QMPRequest (please refer to patch = 11) is > >> >> >> > defined as (mon, id, req) tuple, so at least "id" namespace = is > >> >> >> > per-monitor. > >> >> >> > > >> >> >> >> > And is the dispatching going > >> >> >> >> > to be processed even if the client is disconnected, and a= re new > >> >> >> >> > clients going to receive the replies from previous client= s > >> >> >> >> > commands? > >> >> >> > > >> >> >> > [1] > >> >> >> > > >> >> >> > (will discuss together below) > >> >> >> > > >> >> >> >> > I > >> >> >> >> > believe there should be a per-client context, so there wo= n't be "id" > >> >> >> >> > request conflicts. > >> >> >> > > >> >> >> > I'd say I am not familiar with this "client" idea, since aft= er all > >> >> >> > IMHO one monitor is currently designed to mostly work with a= single > >> >> >> > client. Say, unix sockets, telnet, all these backends are on= ly single > >> >> >> > channeled, and one monitor instance can only work with one c= lient at a > >> >> >> > time. Then do we really need to add this client layer upon = it? IMHO > >> >> >> > the user can just provide more monitors if they wants more c= lients > >> >> >> > (and at least these clients should know the existance of the= others or > >> >> >> > there might be problem, otherwise user2 will fail a migratio= n, finally > >> >> >> > noticed that user1 has already triggered one), and the user = should > >> >> >> > manage them well. > >> >> >> > >> >> >> qemu should support a management layer / libvirt restart/recon= nect. > >> >> >> Afaik, it mostly work today. There might be a cases where libv= irt can > >> >> >> be confused if it receives a reply from a previous connection = command, > >> >> >> but due to the sync processing of the chardev, I am not sure y= ou can > >> >> >> get in this situation. By adding "oob" commands and queuing, = the > >> >> >> client will have to remember which was the last "id" used, or = it will > >> >> >> create more conflict after a reconnect. > >> >> >> > >> >> >> Imho we should introduce the client/connection concept to avoi= d this > >> >> >> confusion (unexpected reply & per client id space). > >> >> > > >> >> > Hmm I agree that the reconnect feature would be nice, but if so= IMHO > >> >> > instead of throwing responses away when client disconnect, we s= hould > >> >> > really keep them, and when the client reconnects, we queue the > >> >> > responses again. > >> >> > > >> >> > I think we have other quite simple ways to solve the "unexpecte= d > >> >> > reply" and "per-client-id duplication" issues you have mentione= d. > >> >> > > >> >> > Firstly, when client gets unexpected replies ("id" field not in= its > >> >> > own request queue), the client should just ignore that reply, w= hich > >> >> > seems natural to me. > >> >> > >> >> The trouble is that it may legitimately use the same "id" value f= or > >> >> new requests. And I don't see a simple way to handle that without > >> >> races. > >> > > >> > Under what circumstances can it reuse the same ID for new requests= ? > >> > Can't we simply tell it not to? > >> > >> I don't see any restriction today in the protocol in connecting with= a > >> new client that may not know anything from a previous client. > > > > Well, it knows it's doing a reconnection. >=20 > If you assume the "same client" reconnects to the monitor, I agree. > But this is a restriction of monitor usage. In monitor_qmp_event(), we can empty the request queue when got CHR_EVENT_CLOSED. Would that be a solution? --=20 Peter Xu