Re: [RFC PATCH v3 3/3] migration: Add fd to FileMigrationArgs

[Daniel P . Berrangé]

On Fri, Mar 15, 2024 at 12:20:40AM -0300, Fabiano Rosas wrote:
> The fd: URI has supported migration to a file or socket since before
> QEMU 8.2. In 8.2 we added the file: URI that supported migration to a
> file. So now we have two ways (three if you count exec:>cat) to
> migrate to a file. Fine.
> 
> However,
> 
> In 8.2 we also added the new qmp_migrate API that uses a JSON channel
> list instead of the URI. It added two migration transports SOCKET and
> FILE. It was decided that the new API would classify the fd migration
> as a type of socket migration, neglecting the fact that the fd.c code
> also supported file migrations.
> 
> In 9.0 we're adding support for fd + multifd + mapped-ram, which is
> tied to the file migration. This was implemented in fd.c, which is
> only reachable when the SOCKET address type is used.
> 
> The result of this is that we're asking users of the new API to create   (1)
> something called a "socket" to perform migration to a plain file. And
> creating something called a "file" provides no way of passing in a
> file descriptor. This is confusing.

The 'file:' protocol eventually calls into qemu_open, and this
transparently allows for FD passing using /dev/fdset/NNN syntax
to pass in FDs. 


> diff --git a/qapi/migration.json b/qapi/migration.json
> index aa1b39bce1..37f4b9c6fb 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -1656,13 +1656,20 @@
>  #
>  # @filename: The file to receive the migration stream
>  #
> +# @fd: A file descriptor name or number.  File descriptors must be
> +#     first added with the 'getfd' command. (since 9.0).
> +#
>  # @offset: The file offset where the migration stream will start
>  #
> +# Since 9.0, all members are optional, but at least one of @filename
> +# or @fd are required.
> +#
>  # Since: 8.2
>  ##
>  { 'struct': 'FileMigrationArgs',
> -  'data': { 'filename': 'str',
> -            'offset': 'uint64' } }
> +  'data': { '*filename': 'str',
> +            '*fd': 'str',
> +            '*offset': 'uint64' } }

Adding 'fd' here is not desirable, because 'filename' is
resolved via qemu_open which allows for FD passing without
introducing any new syntax in interfaces which take filenames.

With regards,
Daniel
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