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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: Tue, 28 Jun 2016 15:37:29 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 2016年06月28日 14:48, Michael S. Tsirkin wrote:
On Thu, Jun 23, 2016 at 12:46:46AM +0800, 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.

I think libvirt will have to bind it.
Passing in an unbound socket wouldn't make sense.
Yes, but libvirt only cares about the creation for all sockets, not sure if this will break the rule and make libvirt considering more beyond it's responsiblility, actually the 'opaque' parameter in the command line is ok for qemu to get the path info and bind it.

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 socket?

/dev/fdset/ is QEMU syntax to pass fd numbers where path is normally
I see, thanks.

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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