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Date: Fri, 22 May 2009 21:10:41 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] Passing fds through a qemu char device
References: <4A172800.8080406@cs.ualberta.ca>
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To: Cam Macdonell <cam@cs.ualberta.ca>
Cc: qemu-devel@nongnu.org

Cam Macdonell wrote:
>
> Hi,
>
> I'm trying to pass eventfds between qemu processes and I'm trying to 
> use a qemu char device to do this.  AFAICT with a qemu_chr_device the 
> polling handler only has support to call read()/write(), not 
> send/recvmsg() which are necessary to send and receive fds with 
> SCM_RIGHTS through the socket.  Is this true or is there support in 
> Qemu for recvmsg() that I'm not seeing?

The TCP/Unix implementations for CharDriverState use send/recv.  You can 
convert the recv path to always use recvmsg() and introduce an 
qemu_chr_ioctl() to pass/recv fds.

You can be smart about storing the recv'd fd in the char driver state 
and that would allow qemu_chr_ioctl(CHR_IOCTL_GET_FD) to not block.  
qemu_chr_ioctl(CHR_IOCTL_SET_FD) would set the fd to be sent for the 
next qemu_chr_write() operation.

Just a thought, other interfaces could also work.

Regards,

Anthony Liguori