From: Christophe de Dinechin <dinechin@redhat.com>
To: Kevin Wolf <kwolf@redhat.com>
Cc: "Peter Maydell" <peter.maydell@linaro.org>,
"Daniel P. Berrange" <berrange@redhat.com>,
"Denis V. Lunev" <den@virtuozzo.com>,
"Cleber Rosa" <cleber@redhat.com>,
"Stefan Hajnoczi" <stefanha@gmail.com>,
"Markus Armbruster" <armbru@redhat.com>,
"Eduardo Habkost" <ehabkost@redhat.com>,
qemu-devel <qemu-devel@nongnu.org>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Michal Privoznik" <mprivozn@redhat.com>,
"Marc-André Lureau" <marcandre.lureau@redhat.com>,
"John Snow" <jsnow@redhat.com>,
"Dominik Csapak" <d.csapak@proxmox.com>
Subject: Re: Making QEMU easier for management tools and applications
Date: Tue, 7 Jan 2020 18:53:13 +0100 [thread overview]
Message-ID: <AE6AD21A-0DAD-4589-BAC0-C203B6F8A0D9@redhat.com> (raw)
In-Reply-To: <20200107125745.GJ4076@linux.fritz.box>
> On 7 Jan 2020, at 13:57, Kevin Wolf <kwolf@redhat.com> wrote:
>
> Am 07.01.2020 um 11:55 hat Michal Privoznik geschrieben:
>> On 1/7/20 10:36 AM, Kevin Wolf wrote:
>>> The easy way out would be tying libvirt to a specific QEMU version. And
>>> I'm only half joking.
>>>
>>> If libvirt didn't exist yet and we needed a management library for QEMU,
>>> what we would build now would probably not look much like libvirt looks
>>> today. We wouldn't try to have basic support for every hypervisor out
>>> there, but integrate it much closer with QEMU and avoid much of the
>>> backwards compatibility requirements that the interface between QEMU and
>>> libvirt has (which requires us to deal with compatibility twice for
>>> everything).
>>
>> By doing this, you would force your consumers to implement compatibility
>> layer each on their own. Freshly finished blockdev is a beautiful example -
>> OpenStack, oVirt and whatnot - they all are/can use blockdev without even
>> noticing, because the API provided by libvirt is stable and provides
>> abstraction, i.e. you don't need to change anything in your domain XML to
>> use blockdev.
>
> Yes and no.
>
> You could still keep using the same abstraction that libvirt has always
> used while doing this. What my imaginary newly written management
> library would do differently isn't necessarily the interface between
> libvirt and applications, but getting rid of backwards compatibility
> requirements in the interface between QEMU and libvirt.
>
> But of course, blockdev isn't even a feature per se. It's getting the
> abstraction right so that it's actually abstract enough to represent
> everything. As long as libvirt keeps using an abstraction that is based
> on simplistic setups, it won't be able to expose the full feature set of
> QEMU. This is less than satisfying. In the long run, libvirt will have
> to extend its abstraction to make full use of new features either way.
>
>> Of course, you can apply the argument one more time and have mgmt
>> application tied to a specific version of qemu. But even that is not
>> good enough, because with backports version is just meaningless
>> number.
>
> I think this would be too much indeed.
>
>>> Maybe it would even be part of the QEMU repository, allowing a single
>>> patch series to implement a new feature in the system emulator and
>>> expose it in the API immediately instead of waiting for the next QEMU
>>> release before libvirt can even think about implementing support for it.
>>
>> Thing is, it's not just qmp that a mgmt application has to master, it's also
>> process managing (and with growing number of helper binaries this is not as
>> trivial as fork() + exec()). This would need to be the bare minimum your API
>> layer has to provide to be consumable by anybody.
>> But then you have some advanced subsystems to take care of (CGroups,
>> SELinux, etc.) which are used heavily by OpenStack. oVirt and friends.
>
> Someone has to do this anyway. Note that here I'm still talking about
> the hypothetical case where no libvirt existed yet.
>
> If we cared only about OpenStack, oVirt and friends, this would still
> all be QEMU-based, so not a big problem to have it tied to QEMU.
>
> I'm not sure what this looks like in practice in libvirt: Are these
> components shared between multiple hypervisor interfaces or is it only
> for QEMU anyway?
>
> If multiple hypervisors make use of it, how crazy would it be to imagine
> reversing which project consumes which? Instead of having the libvirt
> core consume the hypervisor-specific sublibraries, could a QEMU-specific
> part live closer to QEMU and consume the libvirt core as an external
> library?
>
> I guess much of what I write in this thread is pure heresy. :-)
> Maybe most of it isn't even useful. But maybe there is an idea or two in
> it that are worth having a closer look at.
Well, I don’t know if it is heresy, but at least as far as sandboxing / jailing
is concerned, I suggested something similar in earlier iterations of
the discussion. My way to say that I am part of your heresy, I guess.
>
>>> So should libvirt move in that direction? Do people actually still make
>>> much use of its multi-hypervisor nature, or would it make sense to split
>>> it into separate libraries for each hypervisor that can be much tighter
>>> integrated with (a specific version of) the respective hypervisor?
>>
>> Truth to be told, I don't think libvirt is held back by its attempts to
>> provide hypervisor agnostic APIs. Sure, it leads to some weirdness (e.g.
>> different naming in libvirt and qemu), but as a libvirt developer I don't
>> remember feeling blocked by this multi-hypervisor nature (not to mention
>> that this has saved us couple of times).
>
> I would imagine so, because the problem doesn't become visible in the
> daily work, but only in the bigger picture: The other hypervisors are
> what prevent libvirt from being more tightly intergrated with QEMU.
>
> This means that there is a boundary between QEMU and libvirt that makes
> it really slow to get new features to the user. And both QEMU and
> libvirt waste a lot of time for maintaining backwards compatibility in
> things that wouldn't necessarily have to be stable interfaces if the
> management library were developed in lockstep with QEMU.
>
>> Also, it would be not correct to think that a feature is implemented for all
>> hypervisors in libvirt. I mean, when implementing a feature I usually
>> implement it only for qemu driver and don't even look at other drivers
>> (unless I'm doing a change in a core that causes build failures). On the
>> other hand, I do sometimes review patches posted by developers from other
>> companies which have interest in different hypervisors (e.g. there is a SUSE
>> guy working on LXC driver, and another SUSE guy working on libxenlight
>> (basically Xen)), so I do spend some time not working on qemu driver, but
>> I'd say it's negligible.
>
> Time spent on non-QEMU isn't really my concern. Time spent maintaining
> stable interface between QEMU and libvirt, and time spent waiting for
> QEMU releases before libvirt development starts are my concern.
>
> Kevin
next prev parent reply other threads:[~2020-01-07 17:54 UTC|newest]
Thread overview: 183+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-20 16:13 Making QEMU easier for management tools and applications Stefan Hajnoczi
2019-12-20 21:07 ` Richard W.M. Jones
2020-01-02 11:26 ` Stefan Hajnoczi
2019-12-21 9:02 ` Markus Armbruster
2019-12-23 15:04 ` Michal Prívozník
2020-01-07 9:36 ` Kevin Wolf
2020-01-07 10:55 ` Michal Privoznik
2020-01-07 12:57 ` Kevin Wolf
2020-01-07 17:53 ` Christophe de Dinechin [this message]
2019-12-24 13:41 ` Daniel P. Berrangé
2020-01-22 22:28 ` John Snow
2020-01-23 7:19 ` Markus Armbruster
2020-01-23 17:58 ` John Snow
2020-01-23 19:01 ` Daniel P. Berrangé
2020-01-23 21:07 ` John Snow
2020-01-24 7:59 ` Markus Armbruster
2020-01-24 10:27 ` Daniel P. Berrangé
2020-01-24 14:38 ` Kevin Wolf
2020-01-24 18:23 ` John Snow
2020-01-24 18:30 ` Dr. David Alan Gilbert
2020-01-24 18:48 ` John Snow
2020-01-24 18:52 ` Dr. David Alan Gilbert
2020-01-24 18:58 ` John Snow
2020-01-25 10:18 ` Markus Armbruster
2020-01-27 10:18 ` Daniel P. Berrangé
2020-01-27 12:48 ` Markus Armbruster
2020-01-27 11:56 ` Kevin Wolf
2020-01-27 12:04 ` Peter Maydell
2020-01-27 20:11 ` John Snow
2020-01-27 22:38 ` Paolo Bonzini
2020-01-28 0:37 ` John Snow
2020-01-28 10:16 ` Daniel P. Berrangé
2020-01-28 10:39 ` Kevin Wolf
2020-01-28 15:36 ` Markus Armbruster
2020-01-31 12:25 ` Eric Blake
2020-01-28 10:28 ` Kevin Wolf
2020-01-28 12:36 ` Markus Armbruster
2020-01-28 12:54 ` Kevin Wolf
2020-01-28 13:45 ` Gerd Hoffmann
2020-01-31 6:50 ` Markus Armbruster
2020-01-31 7:48 ` Paolo Bonzini
2020-01-31 8:09 ` Markus Armbruster
2020-02-03 20:07 ` Andrea Bolognani
2020-02-04 9:58 ` Markus Armbruster
2020-01-31 12:27 ` Eric Blake
2020-02-02 9:21 ` Kevin Wolf
2020-02-02 10:44 ` Paolo Bonzini
2020-02-03 6:20 ` Markus Armbruster
2020-02-03 8:48 ` Markus Armbruster
2020-01-27 20:12 ` Dr. David Alan Gilbert
2020-01-24 20:34 ` John Snow
2020-01-27 8:35 ` Gerd Hoffmann
2020-01-27 12:13 ` Kevin Wolf
2020-01-27 16:18 ` Gerd Hoffmann
2020-01-24 9:50 ` Daniel P. Berrangé
2020-01-25 11:52 ` Paolo Bonzini
2020-01-27 10:05 ` Daniel P. Berrangé
2020-01-27 8:25 ` Tooling to help humans use JSON (was: Making QEMU easier for management tools and applications) Markus Armbruster
2020-01-27 9:06 ` Making QEMU easier for management tools and applications Markus Armbruster
2020-01-27 10:00 ` Daniel P. Berrangé
2020-01-27 14:35 ` Kevin Wolf
2020-01-27 20:29 ` Dr. David Alan Gilbert
2020-01-28 10:59 ` Kevin Wolf
2020-02-05 13:09 ` Kevin Wolf
2020-02-05 19:09 ` qmp-shell for GSoC/Outreachy? (Was: Re: Making QEMU easier for management tools and applications) John Snow
2020-02-05 19:49 ` Dr. David Alan Gilbert
2020-02-06 9:40 ` qmp-shell for GSoC/Outreachy? Markus Armbruster
2020-02-06 10:09 ` Daniel P. Berrangé
2020-02-06 12:11 ` Markus Armbruster
2020-02-06 12:15 ` Daniel P. Berrangé
2020-02-06 18:02 ` Dr. David Alan Gilbert
2020-02-07 21:03 ` John Snow
2020-02-08 7:17 ` Markus Armbruster
2020-02-06 14:21 ` Kevin Wolf
2020-02-06 18:26 ` Dr. David Alan Gilbert
2020-02-07 10:49 ` Kevin Wolf
2020-02-07 21:23 ` John Snow
2020-02-08 7:25 ` Markus Armbruster
2020-02-10 11:59 ` Kevin Wolf
2020-02-10 12:26 ` Kevin Wolf
2020-02-06 18:18 ` Dr. David Alan Gilbert
2020-02-07 7:47 ` Markus Armbruster
2020-02-07 21:31 ` Eric Blake
2020-02-08 7:34 ` Markus Armbruster
2020-02-07 21:56 ` John Snow
2020-02-07 20:56 ` John Snow
2020-01-27 20:59 ` Making QEMU easier for management tools and applications John Snow
2020-01-28 10:16 ` Markus Armbruster
2020-01-28 19:21 ` John Snow
2020-01-24 6:38 ` Markus Armbruster
2020-01-25 22:34 ` Christophe de Dinechin
2020-01-25 11:55 ` Paolo Bonzini
2020-01-02 14:47 ` Stefan Hajnoczi
2020-01-16 11:03 ` Kashyap Chamarthy
2020-01-20 9:55 ` Stefan Hajnoczi
2020-01-20 13:57 ` Kashyap Chamarthy
2020-01-25 11:41 ` Paolo Bonzini
2020-01-27 19:41 ` John Snow
2020-01-02 15:05 ` Dr. David Alan Gilbert
2020-01-13 13:44 ` Markus Armbruster
2019-12-24 13:00 ` Daniel P. Berrangé
2020-01-02 14:22 ` Stefan Hajnoczi
2020-01-22 22:42 ` John Snow
2020-01-23 7:21 ` Markus Armbruster
2020-01-23 10:27 ` Daniel P. Berrangé
2020-01-23 18:13 ` John Snow
2020-01-23 19:12 ` Daniel P. Berrangé
2020-01-02 15:10 ` Dr. David Alan Gilbert
2020-01-07 17:11 ` Christophe de Dinechin
2020-01-08 10:43 ` Kevin Wolf
2020-01-08 11:40 ` Christophe de Dinechin
2020-01-08 13:38 ` Kevin Wolf
2020-01-14 13:04 ` Markus Armbruster
2020-01-14 17:31 ` Christophe de Dinechin
2020-01-15 9:20 ` Markus Armbruster
2020-01-15 9:34 ` Christophe de Dinechin
2020-01-15 12:15 ` Markus Armbruster
2020-01-15 12:19 ` Daniel P. Berrangé
2020-01-15 14:02 ` Markus Armbruster
2020-01-30 21:09 ` Improving QOM documentation [Was: Re: Making QEMU easier for management tools and applications] Kashyap Chamarthy
2020-01-31 6:11 ` Markus Armbruster
2020-01-31 7:46 ` Paolo Bonzini
2020-01-31 15:37 ` Christophe de Dinechin
2020-01-31 16:28 ` Paolo Bonzini
2020-01-31 9:50 ` Kashyap Chamarthy
2020-01-31 10:35 ` Peter Maydell
2020-01-31 11:02 ` Paolo Bonzini
2020-01-31 15:22 ` Kashyap Chamarthy
2020-01-31 17:23 ` Markus Armbruster
2020-02-03 8:56 ` Paolo Bonzini
2020-02-03 9:54 ` Markus Armbruster
2020-02-03 15:21 ` Paolo Bonzini
2020-02-04 8:42 ` Markus Armbruster
2020-01-31 16:39 ` Markus Armbruster
2020-01-20 10:08 ` Making QEMU easier for management tools and applications Stefan Hajnoczi
2020-01-21 5:42 ` Markus Armbruster
2020-01-21 11:32 ` Stefan Hajnoczi
2020-01-21 12:03 ` Marc-André Lureau
2020-01-21 13:36 ` Integrating QOM into QAPI (was: Making QEMU easier for management tools and applications) Markus Armbruster
2020-01-21 14:36 ` Daniel P. Berrangé
2020-01-21 15:01 ` Integrating QOM into QAPI Markus Armbruster
2020-01-21 15:11 ` Marc-André Lureau
2020-01-21 16:21 ` Peter Maydell
2020-01-22 5:16 ` Getting whole-tree patches reviewed and merged (was: Integrating QOM into QAPI) Markus Armbruster
2020-02-07 21:53 ` Getting whole-tree patches reviewed and merged Eric Blake
2020-02-10 11:26 ` Paolo Bonzini
2020-02-10 16:04 ` Markus Armbruster
2020-02-10 16:12 ` Peter Maydell
2020-01-22 10:50 ` Integrating QOM into QAPI Alex Bennée
2020-01-22 12:24 ` Markus Armbruster
2020-01-22 12:42 ` Marc-André Lureau
2020-01-22 13:28 ` Peter Maydell
2020-01-22 13:32 ` Marc-André Lureau
2020-01-23 7:37 ` Markus Armbruster
2020-01-24 18:32 ` Paolo Bonzini
2020-01-25 4:44 ` Marc-André Lureau
2020-01-25 9:28 ` Paolo Bonzini
2020-01-25 21:25 ` Peter Maydell
2020-01-26 8:09 ` Christophe de Dinechin
2020-01-26 9:11 ` Marc-André Lureau
2020-01-26 16:47 ` Paolo Bonzini
2020-01-27 19:05 ` Christophe de Dinechin
2020-01-27 19:05 ` Christophe de Dinechin
2020-01-26 15:04 ` Peter Maydell
2020-01-27 19:05 ` Christophe de Dinechin
2020-01-28 8:00 ` Markus Armbruster
2020-01-28 10:03 ` Daniel P. Berrangé
2020-01-29 12:42 ` Christophe de Dinechin
2020-01-15 9:35 ` Making QEMU easier for management tools and applications Marc-André Lureau
2020-01-15 12:25 ` Markus Armbruster
2020-01-25 17:18 ` Paolo Bonzini
2020-01-27 9:30 ` Markus Armbruster
2020-01-13 16:30 ` Stefan Hajnoczi
2020-02-04 15:54 ` Summary of " Markus Armbruster
2020-02-05 6:38 ` Markus Armbruster
2020-02-10 10:56 ` Stefan Hajnoczi
2020-02-10 11:01 ` Peter Maydell
2020-02-10 11:08 ` Daniel P. Berrangé
2020-02-10 11:29 ` Peter Maydell
2020-02-10 11:04 ` Paolo Bonzini
2020-02-10 16:43 ` Markus Armbruster
2020-02-12 13:54 ` Stefan Hajnoczi
2020-02-12 14:03 ` Daniel P. Berrangé
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