Re: [PATCH 2/3] migration: Remove unused zero-blocks capability

[Peter Xu <peterx@redhat.com>]

On Thu, Sep 19, 2024 at 01:00:34PM +0000, Dr. David Alan Gilbert wrote:
> * Fabiano Rosas (farosas@suse.de) wrote:
> > Markus Armbruster <armbru@redhat.com> writes:
> > 
> > > Peter Xu <peterx@redhat.com> writes:
> > >
> > >> On Wed, Sep 18, 2024 at 07:52:56AM +0200, Markus Armbruster wrote:
> > >>> dave@treblig.org writes:
> > >>> 
> > >>> > From: "Dr. David Alan Gilbert" <dave@treblig.org>
> > >>> >
> > >>> > migrate_zero_blocks is unused since
> > >>> >   eef0bae3a7 ("migration: Remove block migration")
> > >>> >
> > >>> > Remove it.
> > >>> > That whole zero-blocks capability was just for old-school
> > >>> > block migration anyway.
> > >>> >
> > >>> > Remove the capability as well.
> > >>> >
> > >>> > Signed-off-by: Dr. David Alan Gilbert <dave@treblig.org>
> > >>> > ---
> > >>> >  migration/options.c |  8 --------
> > >>> >  migration/options.h |  1 -
> > >>> >  qapi/migration.json | 10 +---------
> > >>> >  3 files changed, 1 insertion(+), 18 deletions(-)
> > >>> >
> > >>> > diff --git a/migration/options.c b/migration/options.c
> > >>> > index 9460c5dee9..997e060612 100644
> > >>> > --- a/migration/options.c
> > >>> > +++ b/migration/options.c
> > >>> > @@ -177,7 +177,6 @@ Property migration_properties[] = {
> > >>> >      DEFINE_PROP_MIG_CAP("x-xbzrle", MIGRATION_CAPABILITY_XBZRLE),
> > >>> >      DEFINE_PROP_MIG_CAP("x-rdma-pin-all", MIGRATION_CAPABILITY_RDMA_PIN_ALL),
> > >>> >      DEFINE_PROP_MIG_CAP("x-auto-converge", MIGRATION_CAPABILITY_AUTO_CONVERGE),
> > >>> > -    DEFINE_PROP_MIG_CAP("x-zero-blocks", MIGRATION_CAPABILITY_ZERO_BLOCKS),
> > >>> >      DEFINE_PROP_MIG_CAP("x-events", MIGRATION_CAPABILITY_EVENTS),
> > >>> >      DEFINE_PROP_MIG_CAP("x-postcopy-ram", MIGRATION_CAPABILITY_POSTCOPY_RAM),
> > >>> >      DEFINE_PROP_MIG_CAP("x-postcopy-preempt",
> > >>> 
> > >>> Property of (pseudo-)device "migration".  The "x-" prefix suggests we
> > >>> expect management software not to rely on it.  Okay.
> > >>> 
> > >>> [...]
> > >>> 
> > >>> > diff --git a/qapi/migration.json b/qapi/migration.json
> > >>> > index b66cccf107..82d0fc962e 100644
> > >>> > --- a/qapi/migration.json
> > >>> > +++ b/qapi/migration.json
> > >>> > @@ -389,13 +389,6 @@
> > >>> >  #     footprint is mlock()'d on demand or all at once.  Refer to
> > >>> >  #     docs/rdma.txt for usage.  Disabled by default.  (since 2.0)
> > >>> >  #
> > >>> > -# @zero-blocks: During storage migration encode blocks of zeroes
> > >>> > -#     efficiently.  This essentially saves 1MB of zeroes per block on
> > >>> > -#     the wire.  Enabling requires source and target VM to support
> > >>> > -#     this feature.  To enable it is sufficient to enable the
> > >>> > -#     capability on the source VM.  The feature is disabled by
> > >>> > -#     default.  (since 1.6)
> > >>> > -#
> > >>> >  # @events: generate events for each migration state change (since 2.4)
> > >>> >  #
> > >>> >  # @auto-converge: If enabled, QEMU will automatically throttle down
> > >>> > @@ -483,7 +476,7 @@
> > >>> >  # Since: 1.2
> > >>> >  ##
> > >>> >  { 'enum': 'MigrationCapability',
> > >>> > -  'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
> > >>> > +  'data': ['xbzrle', 'rdma-pin-all', 'auto-converge',
> > >>> >             'events', 'postcopy-ram',
> > >>> >             { 'name': 'x-colo', 'features': [ 'unstable' ] },
> > >>> >             'release-ram',
> > >>> 
> > >>> This is used by migrate-set-capabilities and query-migrate-capabilities,
> > >>> via ['MigrationCapabilityStatus'].
> > >>> 
> > >>> query-migrate-capabilities is unaffected: it couldn't return zero-blocks
> > >>> anymore even before the patch.
> > >>> 
> > >>> migrate-set-capabilities changes incompatibly, I'm afraid.  Before the
> > >>> patch:
> > >>> 
> > >>>     {"execute": "migrate-set-capabilities", "arguments": {"capabilities": [{"capability": "zero-blocks", "state": true}]}}
> > >>>     {"return": {}}
> > >>> 
> > >>> Afterwards:
> > >>> 
> > >>>     {"error": {"class": "GenericError", "desc": "Parameter 'capability' does not accept value 'zero-blocks'"}}
> > >>> 
> > >>> If we had somehow rejected the capability when it made no sense,
> > >>> removing it now it never makes sense would be obviously fine.
> > >>> 
> > >>> The straight & narrow path is to deprecate now, remove later.
> > >>
> > >> I wonder whether we can make this one simpler, as IIUC this cap depends on
> > >> the block migration feature, which properly went through the deprecation
> > >> process and got removed in the previous release.
> > >>
> > >> IOW, currently QEMU behaves the same with this cap on/off, ignoring it
> > >> completely.  I think it means the deprecation message (even if we provide
> > >> some for two extra releases..) wouldn't be anything helpful as anyone who
> > >> uses this feature already got affected before this patch.. this feature,
> > >> together with block migration, are simply all gone already?
> > >
> > > We break compatibility for users who supply capability @zero-blocks even
> > > though they are not using block migration.
> > >
> > > Before this patch, the capability is silently ignored.
> > >
> > > Afterwards, we reject it.
> > >
> > > This harmless misuse was *not* affected by our prior removal of block
> > > migration.
> > >
> > > It *is* affected by the proposed removal of the capability.
> > 
> > How does this policy_skip thing works? Could we automatically warn
> > whenever a capability has the 'deprecated' feature in migration.json?
> > 
> > Also, some of the incompatibility errors in migrate_caps_check() could
> > be simplified with something like a new:
> > 'features': [ 'conflicts': [ 'cap1', 'cap2' ] ]
> > to indicate which caps are incompatible between themselves.
> > 
> > >
> > > We either treat this in struct accordance to our rules: deprecate now,
> > > remove later.  Or we bend our them:
> > >
> > >>> If we believe nothing relies on it, we can bend the rules and remove
> > >>> right away.
> > >
> > > Not for me to decide.
> > >
> > 
> > I'm fine either way, but in any case:
> 
> OK, so I'll split my code to just remove the dead function rather than the
> actual capability; Thanks for Fabiano for doing the Deprecation stuff!

I'll wait for a final ACK from Markus, then I can queue below patch
directly into migration-next.

Thanks all!

> 
> Dave
> 
> > -- >8 --
> > >From 3ff313a52e37b8cb407c900d7a1aa266560aebb7 Mon Sep 17 00:00:00 2001
> > From: Fabiano Rosas <farosas@suse.de>
> > Date: Thu, 19 Sep 2024 09:49:44 -0300
> > Subject: [PATCH] migration: Deprecate zero-blocks capability
> > 
> > The zero-blocks capability was meant to be used along with the block
> > migration, which has been removed already in commit eef0bae3a7
> > ("migration: Remove block migration").
> > 
> > Setting zero-blocks is currently a noop, but the outright removal of
> > the capability would cause and error in case some users are still
> > setting it. Put the capability through the deprecation process.
> > 
> > Signed-off-by: Fabiano Rosas <farosas@suse.de>
> > ---
> >  docs/about/deprecated.rst | 6 ++++++
> >  migration/options.c       | 4 ++++
> >  qapi/migration.json       | 5 ++++-
> >  3 files changed, 14 insertions(+), 1 deletion(-)
> > 
> > diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
> > index ed31d4b0b2..47cabb6fcc 100644
> > --- a/docs/about/deprecated.rst
> > +++ b/docs/about/deprecated.rst
> > @@ -476,3 +476,9 @@ usage of providing a file descriptor to a plain file has been
> >  deprecated in favor of explicitly using the ``file:`` URI with the
> >  file descriptor being passed as an ``fdset``. Refer to the ``add-fd``
> >  command documentation for details on the ``fdset`` usage.
> > +
> > +``zero-blocks`` capability (since 9.2)
> > +''''''''''''''''''''''''''''''''''''''
> > +
> > +The ``zero-blocks`` capability was part of the block migration which
> > +doesn't exist anymore since it was removed in QEMU v9.1.
> > diff --git a/migration/options.c b/migration/options.c
> > index 147cd2b8fd..b828bad0d9 100644
> > --- a/migration/options.c
> > +++ b/migration/options.c
> > @@ -457,6 +457,10 @@ bool migrate_caps_check(bool *old_caps, bool *new_caps, Error **errp)
> >      ERRP_GUARD();
> >      MigrationIncomingState *mis = migration_incoming_get_current();
> >  
> > +    if (new_caps[MIGRATION_CAPABILITY_ZERO_BLOCKS]) {
> > +        warn_report("zero-blocks capability is deprecated");
> > +    }
> > +
> >  #ifndef CONFIG_REPLICATION
> >      if (new_caps[MIGRATION_CAPABILITY_X_COLO]) {
> >          error_setg(errp, "QEMU compiled without replication module"
> > diff --git a/qapi/migration.json b/qapi/migration.json
> > index b66cccf107..3af6aa1740 100644
> > --- a/qapi/migration.json
> > +++ b/qapi/migration.json
> > @@ -479,11 +479,14 @@
> >  # Features:
> >  #
> >  # @unstable: Members @x-colo and @x-ignore-shared are experimental.
> > +# @deprecated: Member @zero-blocks is deprecated as being part of
> > +#     block migration which was already removed.
> >  #
> >  # Since: 1.2
> >  ##
> >  { 'enum': 'MigrationCapability',
> > -  'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
> > +  'data': ['xbzrle', 'rdma-pin-all', 'auto-converge',
> > +           { 'name': 'zero-blocks', 'features': [ 'deprecated' ] },
> >             'events', 'postcopy-ram',
> >             { 'name': 'x-colo', 'features': [ 'unstable' ] },
> >             'release-ram',
> > -- 
> > 2.35.3
> > 
> -- 
>  -----Open up your eyes, open up your mind, open up your code -------   
> / Dr. David Alan Gilbert    |       Running GNU/Linux       | Happy  \ 
> \        dave @ treblig.org |                               | In Hex /
>  \ _________________________|_____ http://www.treblig.org   |_______/
> 

-- 
Peter Xu