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

[Markus Armbruster <armbru@redhat.com>]

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.

If we believe nothing relies on it, we can bend the rules and remove
right away.  Missing then: update to docs/about/removed-features.rst.

> @@ -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.