[libvirt] [RFC 2/5] block: add new command line parameter that and protocol description

[Daniel P. Berrange]

On Tue, May 01, 2012 at 04:31:44PM +0100, Stefan Hajnoczi wrote:
> From: Anthony Liguori <aliguori at us.ibm.com>
> 
> Signed-off-by: Anthony Liguori <aliguori at us.ibm.com>
> Signed-off-by: Stefan Hajnoczi <stefanha at linux.vnet.ibm.com>
> ---
>  qemu-options.hx |   42 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
> 
> diff --git a/qemu-options.hx b/qemu-options.hx
> index a169792..ccf4d1d 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -2724,6 +2724,48 @@ DEF("qtest-log", HAS_ARG, QEMU_OPTION_qtest_log,
>      "-qtest-log LOG  specify tracing options\n",
>      QEMU_ARCH_ALL)
>  
> +DEF("open-hook-fd", HAS_ARG, QEMU_OPTION_open_hook_fd,
> +    "-open-hook-fd <fd>\n"
> +    "                delegate opens to external process using <fd>\n", QEMU_ARCH_ALL)
> +STEXI
> + at item -open-hook-fd @var{fd}
> + at findex -open-hook-fd
> +Delegates open()s to an external process using @var<fd> to communicate commands.
> + at var<fd> should be an open Unix Domain socket pipe that file descriptors can be
> +received from.  The protocol the socket uses is a simple request/response initiated
> +by the client.  All integers are in host byte order.  It is assumed that this protocol
> +is only ever used on the same physical machine.  It is currently defined as:
> +
> +u32 message_size
> +u32 command
> +u8  payload[message_size - 8]
> +
> +The contents of payload depend on command.  Currently the following commands are
> +defined:
> +
> +1. QEMU_OPEN (1)
> +
> +The full message will be:
> +
> +u32 message_size
> +u32 command = 1
> +u32 flags (O_ flags defined by libc)
> +u32 mode (mode_t flags as defined by libc)
> +u16 filename_len;
> +u8  filename[filename_len]
> +
> +The server will then respond with:
> +
> +u32 message_size
> +u32 command = 1
> +s32 result

If we're going for a binary protocol, then I'd like to see it defined
based on the XDR specification, so we can auto-generate our data
marshallers/demarshallers using existing tools / libraries and not
have to write something custom by hand. Your spec here is close enough
that it would not be significant work. The changes would be

 - Everything is always big-endian
 - Each field has 4-byte alignment
 - Strings would have a u32 length, and the payload padded with NUL
   to the 4 byte boundary

Daniel.
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