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

[Markus Armbruster <armbru@redhat.com>]

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.

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.

>>              Missing then: update to docs/about/removed-features.rst.
>
> Indeed, we could mention there.
>
> Thanks,
>
>> 
>> > @@ -542,7 +535,6 @@
>> >  #           {"state": false, "capability": "xbzrle"},
>> >  #           {"state": false, "capability": "rdma-pin-all"},
>> >  #           {"state": false, "capability": "auto-converge"},
>> > -#           {"state": false, "capability": "zero-blocks"},
>> >  #           {"state": true, "capability": "events"},
>> >  #           {"state": false, "capability": "postcopy-ram"},
>> >  #           {"state": false, "capability": "x-colo"}
>> 
>> Example for query-migrate-capabilities.  Good.