Re: [PATCH 4/9] migration: Add direct-io parameter

[Fabiano Rosas <farosas@suse.de>]

Markus Armbruster <armbru@redhat.com> writes:

> Peter Xu <peterx@redhat.com> writes:
>
>> On Fri, May 03, 2024 at 05:49:32PM -0300, Fabiano Rosas wrote:
>>> Peter Xu <peterx@redhat.com> writes:
>>> 
>>> > On Fri, Apr 26, 2024 at 11:20:37AM -0300, Fabiano Rosas wrote:
>>> >> Add the direct-io migration parameter that tells the migration code to
>>> >> use O_DIRECT when opening the migration stream file whenever possible.
>>> >> 
>>> >> This is currently only used with the mapped-ram migration that has a
>>> >> clear window guaranteed to perform aligned writes.
>>> >> 
>>> >> Acked-by: Markus Armbruster <armbru@redhat.com>
>>> >> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>>> >> ---
>>> >>  include/qemu/osdep.h           |  2 ++
>>> >>  migration/migration-hmp-cmds.c | 11 +++++++++++
>>> >>  migration/options.c            | 30 ++++++++++++++++++++++++++++++
>>> >>  migration/options.h            |  1 +
>>> >>  qapi/migration.json            | 18 +++++++++++++++---
>>> >>  util/osdep.c                   |  9 +++++++++
>>> >>  6 files changed, 68 insertions(+), 3 deletions(-)
>>> >> 
>>> >> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
>>> >> index c7053cdc2b..645c14a65d 100644
>>> >> --- a/include/qemu/osdep.h
>>> >> +++ b/include/qemu/osdep.h
>>> >> @@ -612,6 +612,8 @@ int qemu_lock_fd_test(int fd, int64_t start, int64_t len, bool exclusive);
>>> >>  bool qemu_has_ofd_lock(void);
>>> >>  #endif
>>> >>  
>>> >> +bool qemu_has_direct_io(void);
>>> >> +
>>> >>  #if defined(__HAIKU__) && defined(__i386__)
>>> >>  #define FMT_pid "%ld"
>>> >>  #elif defined(WIN64)
>>> >> diff --git a/migration/migration-hmp-cmds.c b/migration/migration-hmp-cmds.c
>>> >> index 7e96ae6ffd..8496a2b34e 100644
>>> >> --- a/migration/migration-hmp-cmds.c
>>> >> +++ b/migration/migration-hmp-cmds.c
>>> >> @@ -397,6 +397,13 @@ void hmp_info_migrate_parameters(Monitor *mon, const QDict *qdict)
>>> >>          monitor_printf(mon, "%s: %s\n",
>>> >>              MigrationParameter_str(MIGRATION_PARAMETER_MODE),
>>> >>              qapi_enum_lookup(&MigMode_lookup, params->mode));
>>> >> +
>>> >> +        if (params->has_direct_io) {
>>> >> +            monitor_printf(mon, "%s: %s\n",
>>> >> +                           MigrationParameter_str(
>>> >> +                               MIGRATION_PARAMETER_DIRECT_IO),
>>> >> +                           params->direct_io ? "on" : "off");
>>> >> +        }
>>> >
>>> > This will be the first parameter to optionally display here.  I think it's
>>> > a sign of misuse of has_direct_io field..
>
> Yes.  For similar existing parameters, we do
>
>                assert(params->has_FOO);
>                monitor_printf(mon, "%s: '%s'\n",
>                               MigrationParameter_str(MIGRATION_PARAMETER_FOO),
>                               ... params->FOO as string ...)
>
>>> > IMHO has_direct_io should be best to be kept as "whether direct_io field is
>>> > valid" and that's all of it.  It hopefully shouldn't contain more
>>> > information than that, or otherwise it'll be another small challenge we
>>> > need to overcome when we can remove all these has_* fields, and can also be
>>> > easily overlooked.
>>> 
>>> I don't think I understand why we have those has_* fields. I thought my
>>> usage of 'params->has_direct_io = qemu_has_direct_io()' was the correct
>>> one, i.e. checking whether QEMU has any support for that parameter. Can
>>> you help me out here?
>>
>> Here params is the pointer to "struct MigrationParameters", which is
>> defined in qapi/migration.json.  And we have had "has_*" only because we
>> allow optional fields with asterisks:
>>
>>   { 'struct': 'MigrationParameters',
>>     'data': { '*announce-initial': 'size',
>>               ...
>>               } }
>>
>> So that's why it better only means "whether this field existed", because
>> it's how it is defined.
>>
>> IIRC we (or say, Markus) used to have some attempts deduplicates those
>> *MigrationParameter* things, and if success we have chance to drop has_*
>> fields (in which case we simply always have them; that "has_" makes more
>> sense only if in a QMP session to allow user only specify one or more
>> things if not all).
>
> qapi/migration.json:
>
>     ##
>     # @MigrationParameters:
>     #
> --> # The optional members aren't actually optional.
>     #
>
> In other words, the has_FOO generated for the members of
> MigrationParameters must all be true.
>
> These members became optional when we attempted to de-duplicate
> MigrationParameters and MigrateSetParameters, but failed (see commit
> de63ab61241 "migrate: Share common MigrationParameters struct" and
> commit 1bda8b3c695 "migration: Unshare MigrationParameters struct for
> now").

So what's the blocker for merging MigrationSetParameters and
MigrationParameters? Just the tls_* options?