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Re: [Qemu-devel] [RFC Patch 1/3] chardev: add new socket fd parameter fo

From: Wei Xu
Subject: Re: [Qemu-devel] [RFC Patch 1/3] chardev: add new socket fd parameter for unix socket
Date: Mon, 27 Jun 2016 23:47:28 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

Anyone can help on this, or at least help me with question 2 about how to test it using 'add-fd' command line?

Q2. Does anybody know how can i test it? i'm going to fork a process firstly and create a socket like libvirt, then exec qemu and pass it in, just wondering how can i map the socket to '/dev/fdset/' directory after created the socket? Another issue is if 'set=2' is a right usage for a socket fd, any comment?

Command line:
qemu-kvm -add-fd fd=3,set=2,opaque="/var/run/openvswitch/vhost-user1"
         -chardev socket,id=char0,path=/dev/fdset/3,server

On 2016年06月23日 00:46, Wei Xu wrote:
On 2016年06月22日 23:39, Eric Blake wrote:
On 06/22/2016 09:25 AM, Wei Xu wrote:
There have been comments on this patch, but i forgot adding this
patch to
the list, just forward it again.

When manage VMs via libvirt, qemu ofter runs with limited permission,
thus qemu can't create a file/socket, this patch is to  add a new
parameter 'sockfd' to accept fd opened and passed in from libvirt.

Signed-off-by: Wei Xu <address@hidden>
  qapi-schema.json | 3 ++-
  qemu-char.c      | 3 +++
  2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/qapi-schema.json b/qapi-schema.json
index 8483bdf..e9f0268 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -2921,7 +2921,8 @@
  { 'struct': 'UnixSocketAddress',
    'data': {
-    'path': 'str' } }
+    'path': 'str',
+    'sockfd': 'int32' } }

Missing documentation.

This makes the new 'sockfd' parameter mandatory, but SocketAddress is an
input type.  This is not backwards compatible.  At best, you'd want to
make it optional, but I'm not even convinced you want to add it, since
we already can use the magic /dev/fdset/nnn in 'path' to pass an
arbitrary fd if the underlying code uses qemu_open().

Thanks for commenting it again, i was going to forward it to the list
and ask some questions.:)

Actually i'm going to try the magic way as you suggested, just a few
questions about that.

Command line change should be like this according to my understanding.

Current command line:
-chardev socket,id=char0,path=/var/run/openvswitch/vhost-user1,server

New command line:
qemu-kvm -add-fd fd=3,set=2,opaque="/var/run/openvswitch/vhost-user1"
-chardev socket,id=char0,path=/dev/fdset/3,server

Q1. The 'sockfd' is not used anymore, but looks the 'path' parameter is
still mandatory AFAIK, because it's a unix domain socket, which is
different with a network tcp/udp socket, it works like a pipe file for
local communication only, and the 'path' parameter is a must-have
condition before a real connect could be established, it needs a bind()
operation to a specific path before either connect() or listen(), this
is caused by libvirt only takes the responsibility to creates a socket
and pass the 'fd' in only, there is nothing more about the bind(), thus
i think qemu will have to bind() it by itself, i'm thinking maybe
'opaque' can be used for this case.

Q2. Do you know how can i test it? i'm going to fork a process first and
create a socket like libvirt, then exec qemu and pass it in, just
wondering how can i map it to '/dev/fdset/' directory after created the

Daniel's comment before:
'Path' refers to a UNIX domain socket path, so the code won't be using
qemu_open() - it'll be using the sockets APIs to open/create a UNIX
socket. I don't think qemu_open() would even work with a socket FD,
since it'll be trying to set various modes on the FD via fcntl() that
don't work with sockets AFAIK
Seems what i should call is qemu_socket() to fill in qemu_open(), i
should check if it's started as '/dev/fdset' like in qemu_open(), and
just pick the 'fd' up, is this enough? should i check the modes?


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