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Re: [Xen-devel] [RFC v2 6/9] libxl: Have QEMU save its state to a file descriptor

On Mon, Apr 23, 2018 at 10:20:42AM +0100, Wei Liu wrote:
> On Mon, Apr 16, 2018 at 06:32:24PM +0100, Anthony PERARD wrote:
> > In case QEMU have restricted access to the system, open the file for it,
> > and QEMU will save its state to this file descritor.
> > 
> > Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> I think this is going to break FreeBSD?

If the previous patch works on FreeBSD (Learned to send FD through QMP
to QEMU), this patch shouldn't break anything. Looking at how
libxl__sendmsg_fds() is implemented, and how the receive side is
implemented in QEMU, there doesn't seems to be any difference on
FreeBSD, so I guess it's going to work fine.

> And what is /dev/fdset exactly? I don't seem to have it on my Debian
> workstation (with Linux 4.15). So this change can potentially be broken
> on Linux as well?

/dev/fdset is a QEMU internal API. You can add a file descriptor to a
fd-set via QMP or via the command line. And later, you can give a path
to something in QEMU (maybe a block driver) which refer to a fd-set
created earlier. The path would be "/dev/fdset/$fdset_number". There can
be several fd in a fd-set, qemu will just look for the first one which
have the right permission, like read or write permission.

Anthony PERARD

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