11:04 a.m.
On 12/08/2015 07:59 AM, Daniel P. Berrange wrote:
So for this my plan is to stop using the QEMU 'file' backend
for char
devs and instead pass across a pre-opened file descriptor, connected
to virtlogd. There is no "officially documented" way to pass in a
file descriptor to QEMU chardevs, but since QEMU uses qemu_open(),
we can make use of the fdset feature to achieve this. eg
eg, consider fd 33 is the write end of a pipe file descriptor
I can (in theory) do
-add-fd set=2,fd=33 -chardev file,id=charserial0,path=/dev/fdset/2
Now in practice this doesn't work, because qmp_chardev_open_file()
passes the O_CREAT|O_TRUNC flags in, which means the qemu_open()
call will fail when using the pipe FD pased in via fdsets.
Is it just the O_TRUNC that is failing? If so, there is a recent patch
to add an 'append':true flag that switches O_TRUNC off in favor of O_APPEND:
https://lists.gnu.org/archive/html/qemu-devel/2015-12/msg00762.html
Or is it that the pipe is one-way, but chardev insists on O_RDWR and
fails because it is not two-way?
After more investigation I found it *is* possible to use a socketpair
and a pipe backend though...
-add-fd set=2,fd=33 -chardev pipe,id=charserial0,path=/dev/fdset/2
Yes, a socketpair is bi-directional, so it supports O_RDWR opening.
..because for reasons I don't understand, if QEMU can't open $PATH.in
and $PATH.out, it'll fallback to just opening $PATH in read-write
mode even. AFAICT, this is pretty useless with pipes since they
are unidirectional, but, it works nicely with socketpairs, where
virtlogd has one of the socketpairs and QEMU gets passed the other
via fdset.
Is it something where we'd want to support two pipes, and open
/dev/fdset/2 tied to char.in and /dev/fdset/3 tied to char.out, where
uni-directional pipes are again okay?
I can easily check this works for historical QEMU versions back
to when fdsets support was added to chardevs, but I'm working if
the QEMU maintainers consider this usage acceptable over the long
term, and if so, should we explicitly document it as supported ?
It seems like a bi-directional socketpair as the single endpoint for a
chardev is useful enough to support and document, but I'm not the
maintainer to give final say-so.
If not, should we introduce a more explicit syntax for passing in
a pre-opened FD for chardevs ? eg
-add-fd set=2,fd=33 -chardev fd,id=charserial0,path=/dev/fdset/2
Difference to the line you tried above:
-add-fd set=2,fd=33 -chardev file,id=charserial0,path=/dev/fdset/2
is 'fd' instead of 'file'. But if we're going to add a new protocol,
do
we even need to go through the "/dev/fdset/..." name, or can we just
pass the fd number directly?
Or just make -chardev file,id=charserial0,path=/dev/fdset/2
actually
work ?
I'd lean more to this case - the whole point of fdsets was that we don't
have to add multiple fd protocols; that everyone that understood file
syntax and uses qemu_open() magically gained fd support.
Or something else ?
OpenStack has a further requirement to allow use of the serial port
as an interactive console, at the same time that it is logging to a
file which is something QEMU can't support at all currently. This
essentially means being able to have multiple chardev backends all
connected to the same serial frontend - specifically we would need
a TCP backend and a file backend concurrently. Again this could be
implemented in QEMU, but we'd prefer something that works with
existing QEMU.
This is not too difficult to achieve with virtlogd really. Instead
of using the QEMU 'tcp' or 'unix' chardev protocol, we'd just always
pass QEMU a pre-opened socketpair, and then leave the TCP/UNIX
socket listening to the virtlogd daemon.
This is portable with existing QEMU versions, but the obvious downside
with this is extra copies in the interactive console path. So might it
be worth exploring the posibility of a chardev multiplexor in QEMU. We
would still pass in a pre-opened socketpair to QEMU for the logging side
of things, but would leave the TCP/UNIX socket listening upto QEMU still.
eg should we make something like this work:
-add-fd set=2,fd=33
-chardev pipe,id=charserial0file,path=/dev/fdset/2
-chardev socket,id=charserial0tcp,host=127.0.0.1,port=9999,telnet,server,nowait
-chardev multiplex,id=charserial0,muxA=charserial0file,muxB=charserial1
wouldn't muxB be charserial0tcp (not charserial1)?
-serial isa-serial,chardev=charserial0,id=serial0
But the idea of a multiplex protocol that has multiple data sinks (guest
output copied to all sinks) and a single source (at most one source can
provide input to the guest) makes sense on the surface.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org