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

[Fabiano Rosas <farosas@suse.de>]

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:

-- >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