Re: [Qemu-devel] [PATCH for-2.12] monitor: bind dispatch bh to iohandler context

[Peter Xu]

On Wed, Apr 04, 2018 at 03:22:20PM +0100, Stefan Hajnoczi wrote:
> On Tue, Apr 03, 2018 at 01:01:15PM +0800, Peter Xu wrote:
> > Eric Auger reported the problem days ago that OOB broke ARM when running
> > with libvirt:
> > 
> > http://lists.gnu.org/archive/html/qemu-devel/2018-03/msg06231.html

[1]

> > 
> > This patch fixes the problem.
> > 
> > It's not really needed now since we have turned OOB off now, but it's
> > still a bug fix, and it'll start to work when we turn OOB on for ARM.
> > 
> > The problem was that the monitor dispatcher bottom half was bound to
> > qemu_aio_context, but that context seems to be for block only.
> 
> No, it is not block-only.  iohandler_ctx is for the legacy
> qemu_set_fd_handler() API only and modern code should use
> qemu_aio_context.
> 
> The difference between qemu_aio_context and iohandler_ctx is that
> aio_poll(qemu_aio_context) does not process iohandler_ctx (since it's a
> difference context).  That is the legacy behavior that
> qemu_set_fd_handler() expects and it's implemented by keeping a separate
> iohandler_ctx.
> 
> > For the
> > rest of the QEMU world we should be using iohandler context.  So
> > assigning monitor dispatcher bottom half to that context.
> 
> This patch relies on the side-effect that iohandler_ctx is only called
> later by the main loop, which seems to prevent the crash below.

Actually I thought that's why we need that iohandler_ctx, no?

Any better suggestion would be welcomed...

> 
> What is the actual crash/problem?  You mentioned the GIC, but what does
> that have to do with monitor code crashing?

The actually crash is mentioned above [1].  Please have a look on that
thread for details.

Basic idea is that the QMP command "cont" is run earlier than before,
which breaks the ordering of past, so vcpu threads are running
earlier, even if they are not really fully setup.  Strange things
happen when we want to run un-prepared vcpus.

> 
> > 
> > If without this change, QMP dispatcher might be run even before reaching
> > main loop in block IO path, for example, in a stack like:
> > 
> >         #0  qmp_cont ()
> >         #1  0x00000000006bd210 in qmp_marshal_cont ()
> >         #2  0x0000000000ac05c4 in do_qmp_dispatch ()
> >         #3  0x0000000000ac07a0 in qmp_dispatch ()
> >         #4  0x0000000000472d60 in monitor_qmp_dispatch_one ()
> >         #5  0x000000000047302c in monitor_qmp_bh_dispatcher ()
> >         #6  0x0000000000acf374 in aio_bh_call ()
> >         #7  0x0000000000acf428 in aio_bh_poll ()
> >         #8  0x0000000000ad5110 in aio_poll ()
> >         #9  0x0000000000a08ab8 in blk_prw ()
> >         #10 0x0000000000a091c4 in blk_pread ()
> >         #11 0x0000000000734f94 in pflash_cfi01_realize ()
> >         #12 0x000000000075a3a4 in device_set_realized ()
> >         #13 0x00000000009a26cc in property_set_bool ()
> >         #14 0x00000000009a0a40 in object_property_set ()
> >         #15 0x00000000009a3a08 in object_property_set_qobject ()
> >         #16 0x00000000009a0c8c in object_property_set_bool ()
> >         #17 0x0000000000758f94 in qdev_init_nofail ()
> >         #18 0x000000000058e190 in create_one_flash ()
> >         #19 0x000000000058e2f4 in create_flash ()
> >         #20 0x00000000005902f0 in machvirt_init ()
> >         #21 0x00000000007635cc in machine_run_board_init ()
> >         #22 0x00000000006b135c in main ()
> > 
> > This can cause ARM to crash when used with both OOB capability enabled
> > and libvirt as upper layer, since libvirt will start QEMU with "-S" and
> > the first "cont" command will arrive very early if the context is not
> > correct (which is what above stack shows).  Then, the vcpu threads will
> > start to run right after the qmp_cont() call, even when GICs have not
> > been setup correctly yet (which is done in kvm_arm_machine_init_done()).
> >
> > My sincere thanks to Eric Auger who offered great help during both
> > debugging and verifying the problem.  The ARM test was carried out by
> > applying this patch upon QEMU 2.12.0-rc0 and problem is gone after the
> > patch.
> > 
> > A quick test of mine shows that after this patch applied we can pass all
> > raw iotests even with OOB on by default.
> > 
> > CC: Eric Blake <address@hidden>
> > CC: Markus Armbruster <address@hidden>
> > CC: Stefan Hajnoczi <address@hidden>
> > CC: Fam Zheng <address@hidden>
> > Reported-by: Eric Auger <address@hidden>
> > Tested-by: Eric Auger <address@hidden>
> > Signed-off-by: Peter Xu <address@hidden>
> > ---
> > 
> > This patch will fix all known OOB breakages I know so far, but I think
> > for better safety I'll still keep OOB off, and I'll send another patch
> > to turn default OOB on after 2.12 release.
> > ---
> >  monitor.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/monitor.c b/monitor.c
> > index 51f4cf480f..39f8ee17ba 100644
> > --- a/monitor.c
> > +++ b/monitor.c
> > @@ -4467,7 +4467,7 @@ static void monitor_iothread_init(void)
> >       * have assumption to be run on main loop thread.  It would be
> >       * nice that one day we can remove this assumption in the future.
> >       */
> > -    mon_global.qmp_dispatcher_bh = aio_bh_new(qemu_get_aio_context(),
> > +    mon_global.qmp_dispatcher_bh = aio_bh_new(iohandler_get_aio_context(),
> >                                                monitor_qmp_bh_dispatcher,
> >                                                NULL);
> >  
> > -- 
> > 2.14.3
> > 
> > 



-- 
Peter Xu