From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To: Yury Kotov <yury-kotov@yandex-team.ru>
Cc: Laurent Vivier <lvivier@redhat.com>,
Thomas Huth <thuth@redhat.com>,
Juan Quintela <quintela@redhat.com>,
Markus Armbruster <armbru@redhat.com>,
"open list:All patches CC here" <qemu-devel@nongnu.org>,
"yc-core@yandex-team.ru" <yc-core@yandex-team.ru>,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [Qemu-devel] [PATCH 1/3] migration: Add x-validate-uuid capability
Date: Tue, 3 Sep 2019 18:13:14 +0100 [thread overview]
Message-ID: <20190903171314.GQ2744@work-vm> (raw)
In-Reply-To: <2870661567528763@iva5-c4dd0484b46b.qloud-c.yandex.net>
* Yury Kotov (yury-kotov@yandex-team.ru) wrote:
> 03.09.2019, 14:25, "Dr. David Alan Gilbert" <dgilbert@redhat.com>:
> > * Eric Blake (eblake@redhat.com) wrote:
> >> On 8/27/19 10:36 AM, Yury Kotov wrote:
> >> > 27.08.2019, 17:02, "Eric Blake" <eblake@redhat.com>:
> >> >> On 8/27/19 7:02 AM, Yury Kotov wrote:
> >> >>> This capability realizes simple source validation by UUID.
> >> >>> It's useful for live migration between hosts.
> >> >>>
> >>
> >> >>
> >> >> Any reason why this is marked experimental? It seems useful enough that
> >> >> we could probably just add it as a fully-supported feature (dropping the
> >> >> x- prefix) - but I'll leave that up to the migration maintainers.
> >> >>
> >> >
> >> > I thought that all new capabilities have x- prefix... May be it's really
> >> > unnecessary here, I'm not sure.
> >>
> >> New features that need some testing or possible changes to behavior need
> >> x- to mark them as experimental, so we can make those changes without
> >> worrying about breaking back-compat. But new features that are outright
> >> useful and presumably in their final form, with no further
> >> experimentation needed, can skip going through an x- phase.
> >>
> >> >
> >> >> In fact, do we even need this to be a tunable feature? Why not just
> >> >> always enable it? As long as the UUID is sent in a way that new->old
> >> >> doesn't break the old qemu from receiving the migration stream, and that
> >> >> old->new copes with UUID being absent, then new->new will always benefit
> >> >> from the additional safety check.
> >> >>
> >> >
> >> > In such case we couldn't migrate from e.g. 4.2 to 3.1
> >>
> >> I don't know the migration format enough to know if there is a way for
> >> 4.2 to unconditionally send a UUID as a subsection such that a receiving
> >> 3.1 will ignore the unknown subsection. If so, then you don't need a
> >> knob; if not, then you need something to say whether sending the
> >> subsection is safe (perhaps default on in new machine types, but default
> >> off for machine types that might still be migrated back to 3.1). That's
> >> where I'm hoping the migration experts will chime in.
> >
> > Right; the migration format can't ignore chunks of data; so it does need
> > to know somehow; the choice is either a capability or wiring it to the
> > machine type; my preference is to wire it to the machine type; the
> > arguments are:
> > a) Machine type
> > Pro: libvirt doesn't need to do anything
> > Con: It doesn't protect old machine types
> > It's not really part of the guest state
> >
> > b) Capability
> > Pro: Works on all machine types
> > Con: Libvirt needs to enable it
> >
> > So, no strong preference but I think I prefer (a).
>
> IIUC the (a) option requires to add a piece of code to every machine type.
> This is much more complicated than adding a capability.
Actually it doesn't - you just add a property, default it to true and
then add an entry in hw_compat_4_1 to turn it off for older types.
> If you don't mind, I suggest to keep the current version.
That's OK.
Dave
> >
> > Dave
> >
> >> --
> >> Eric Blake, Principal Software Engineer
> >> Red Hat, Inc. +1-919-301-3226
> >> Virtualization: qemu.org | libvirt.org
> >
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>
> Regards,
> Yury
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
next prev parent reply other threads:[~2019-09-03 17:14 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-27 12:02 [Qemu-devel] [PATCH 0/3] UUID validation during migration Yury Kotov
2019-08-27 12:02 ` [Qemu-devel] [PATCH 1/3] migration: Add x-validate-uuid capability Yury Kotov
2019-08-27 14:01 ` Eric Blake
2019-08-27 15:36 ` Yury Kotov
2019-08-27 16:18 ` Eric Blake
2019-09-03 11:25 ` Dr. David Alan Gilbert
2019-09-03 16:39 ` Yury Kotov
2019-09-03 17:13 ` Dr. David Alan Gilbert [this message]
2019-09-03 11:34 ` Dr. David Alan Gilbert
2019-08-27 12:02 ` [Qemu-devel] [PATCH 2/3] tests/libqtest: Allow to set expected exit status Yury Kotov
2019-08-27 13:52 ` Marc-André Lureau
2019-08-27 15:23 ` Yury Kotov
2019-08-27 14:03 ` Eric Blake
2019-08-27 15:27 ` Yury Kotov
2019-08-27 12:02 ` [Qemu-devel] [PATCH 3/3] tests/migration: Add a test for x-validate-uuid capability Yury Kotov
2019-09-03 11:21 ` [Qemu-devel] [PATCH 0/3] UUID validation during migration Dr. David Alan Gilbert
2019-09-03 11:45 ` Daniel P. Berrangé
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