Libguestfs - Oct 2019 - Re: [PATCH libnbd 3/4] api: Add nbd_connect_socket.

On 10/4/19 2:50 PM, Richard W.M. Jones wrote:
> This allows us to connect directly to a connected socket.  How exactly
> the socket is created and connected to the NBD server is left up to
> the main program.
> 
> The only real complexity in this patch is allowing file descriptors to
> be passed to libnbd APIs.  Luckily in C and Python we can treat them
> exactly as integers, and in OCaml they are also integers but with a
> different type.
> ---
>   generator/generator | 72 ++++++++++++++++++++++++++++++++++++---------
>   lib/connect.c       | 47 +++++++++++++++++++++++++++++
>   2 files changed, 105 insertions(+), 14 deletions(-)
> 
> +  "connect_socket", {
> +    default_call with
> +    args = [ Fd "sock" ]; ret = RErr;
> +    permitted_states = [ Created ];
> +    shortdesc = "connect directly to a connected socket";
> +    longdesc = "\
> +Pass a connected socket (file descriptor) which libnbd will
> +talk to.  The program is responsible for connecting this
> +somehow to an NBD server.  Once the socket is passed to
> +libnbd it is the responsibility of libnbd.  Libnbd may
> +read and write to it, set it to non-blocking, etc., and
> +will finally close it when the handle is closed.  The
> +program must no longer use the socket.";
> +  };
Do we want a mode where we can hand a socket to libnbd that has already 
completed handshake phase and is immediately ready for transaction 
phase?  I've seen posts describing alternative programs that do their 
own handshake before using ioctls to hand an open socket to the kernel 
kvm module, which only needs to know the size and flags associated with 
the socket, to proceed to use it without further negotiation.  Having 
such a mode would allow libnbd instead of the kernel ioctl to be used in 
the same manner.  But having that as a separate API rather than trying 
to cram it into this one is fine.  (And same counterpart question for 
nbdkit: do we want a mode where nbdkit can be handed an already-open 
socket and details about what flags the client side has been told, and 
skip handshake straight into serving client transaction requests)

Series looks great to me.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

On Fri, Oct 04, 2019 at 03:08:15PM -0500, Eric Blake
wrote:
> Do we want a mode where we can hand a socket to libnbd that has
> already completed handshake phase and is immediately ready for
> transaction phase?  I've seen posts describing alternative programs
> that do their own handshake before using ioctls to hand an open
> socket to the kernel kvm module, which only needs to know the size
nbd :-?
> and flags associated with the socket, to proceed to use it without
> further negotiation.  Having such a mode would allow libnbd instead
> of the kernel ioctl to be used in the same manner.  But having that
> as a separate API rather than trying to cram it into this one is
> fine.  (And same counterpart question for nbdkit: do we want a mode
> where nbdkit can be handed an already-open socket and details about
> what flags the client side has been told, and skip handshake
> straight into serving client transaction requests)
I understand this may be necessary for the kernel module.  If we can
be sure it really has a use-case [nbd-runner?] then yes.
> Series looks great to me.
It makes the fuzzer run about 10% faster (because of faster
execs/sec), so it's not a huge win but definitely worth it I think.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org

On 10/4/19 3:33 PM, Richard W.M. Jones wrote:
> On Fri, Oct 04, 2019 at 03:08:15PM -0500, Eric Blake wrote:
>> Do we want a mode where we can hand a socket to libnbd that has
>> already completed handshake phase and is immediately ready for
>> transaction phase?  I've seen posts describing alternative programs
>> that do their own handshake before using ioctls to hand an open
>> socket to the kernel kvm module, which only needs to know the size
> 
> nbd :-?
Yes.  Thanks for catching my typo.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org