Re: [PATCH v2 14/18] migration: Add documentation for fdset with multifd + file

[Peter Xu]

On Thu, May 23, 2024 at 04:05:44PM -0300, Fabiano Rosas wrote:
> With the last few changes to the fdset infrastructure, we now allow
> multifd to use an fdset when migrating to a file. This is useful for
> the scenario where the management layer wants to have control over the
> migration file.
> 
> By receiving the file descriptors directly, QEMU can delegate some
> high level operating system operations to the management layer (such
> as mandatory access control). The management layer might also want to
> add its own headers before the migration stream.
> 
> Document the "file:/dev/fdset/#" syntax for the multifd migration with
> mapped-ram. The requirements for the fdset mechanism are:
> 
> - the fdset must contain two fds that are not duplicates between
>   themselves;
> 
> - if direct-io is to be used, exactly one of the fds must have the
>   O_DIRECT flag set;
> 
> - the file must be opened with WRONLY on the migration source side;
> 
> - the file must be opened with RDONLY on the migration destination
>   side.
> 
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
>  docs/devel/migration/main.rst       | 24 +++++++++++++++++++-----
>  docs/devel/migration/mapped-ram.rst |  6 +++++-
>  2 files changed, 24 insertions(+), 6 deletions(-)
> 
> diff --git a/docs/devel/migration/main.rst b/docs/devel/migration/main.rst
> index 495cdcb112..784c899dca 100644
> --- a/docs/devel/migration/main.rst
> +++ b/docs/devel/migration/main.rst
> @@ -47,11 +47,25 @@ over any transport.
>    QEMU interference. Note that QEMU does not flush cached file
>    data/metadata at the end of migration.
>  
> -In addition, support is included for migration using RDMA, which
> -transports the page data using ``RDMA``, where the hardware takes care of
> -transporting the pages, and the load on the CPU is much lower.  While the
> -internals of RDMA migration are a bit different, this isn't really visible
> -outside the RAM migration code.
> +  The file migration also supports using a file that has already been
> +  opened. A set of file descriptors is passed to QEMU via an "fdset"
> +  (see add-fd QMP command documentation). This method allows a
> +  management application to have control over the migration file
> +  opening operation. There are, however, strict requirements to this
> +  interface if the multifd capability is enabled:
> +
> +    - the fdset must contain two file descriptors that are not
> +      duplicates between themselves;
> +    - if the direct-io capability is to be used, exactly one of the
> +      file descriptors must have the O_DIRECT flag set;
> +    - the file must be opened with WRONLY on the migration source side
> +      and RDONLY on the migration destination side.
> +
> +- rdma migration: support is included for migration using RDMA, which
> +  transports the page data using ``RDMA``, where the hardware takes
> +  care of transporting the pages, and the load on the CPU is much
> +  lower.  While the internals of RDMA migration are a bit different,
> +  this isn't really visible outside the RAM migration code.
>  
>  All these migration protocols use the same infrastructure to
>  save/restore state devices.  This infrastructure is shared with the
> diff --git a/docs/devel/migration/mapped-ram.rst 
> b/docs/devel/migration/mapped-ram.rst
> index fa4cefd9fc..e6505511f0 100644
> --- a/docs/devel/migration/mapped-ram.rst
> +++ b/docs/devel/migration/mapped-ram.rst
> @@ -16,7 +16,7 @@ location in the file, rather than constantly being added to 
> a
>  sequential stream. Having the pages at fixed offsets also allows the
>  usage of O_DIRECT for save/restore of the migration stream as the
>  pages are ensured to be written respecting O_DIRECT alignment
> -restrictions (direct-io support not yet implemented).
> +restrictions.
>  
>  Usage
>  -----
> @@ -35,6 +35,10 @@ Use a ``file:`` URL for migration:
>  Mapped-ram migration is best done non-live, i.e. by stopping the VM on
>  the source side before migrating.
>  
> +For best performance enable the ``direct-io`` capability as well:

Parameter?

> +
> +    ``migrate_set_capability direct-io on``

migrate_set_parameters?

If with that fixed:

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu