Re: [Qemu-devel] [PATCH] [RFC] aio/async: Add timed bottom-halves

[Stefan Hajnoczi]

On Tue, Jul 16, 2013 at 10:24:38PM +0100, Alex Bligh wrote:
> --On 16 July 2013 18:55:15 +0200 Paolo Bonzini <address@hidden> wrote:
> 
> >>What do you think? In the end I thought the schedule_bh_at stuff
> >>was simpler.
> >
> >It is simpler, but I'm not sure it is the right API.  Of course, if
> >Kevin and Stefan says it is, I have no problem with that.
> 
> For the sake of having something to comment on, I just sent v3 of this
> patch to the list. This is basically a 'minimal change' version that
> fixes the issue with aio_poll (I think). It passes make check.

I would prefer to stick with QEMUTimer instead of introducing an
AioContext-specific concept that does something very similar.

This can be done by introducing a per-AioContext QEMUClock.  Legacy
QEMUTimers will not run during aio_poll() because they are associated
with vm_clock, host_clock, or rt_clock.  Only QEMUTimers associated with
this AioContext's aio_ctx_clock will run.

In other words, the main loop will run vm_clock, host_clock, and
rt_clock timers.  The AioContext will run its aio_ctx_clock timers.

A few notes about QEMUTimer and QEMUClock:

 * QEMUClock can be enabled/disabled.  Disabled clocks suppress timer
   expiration until re-enabled.

 * QEMUClock can use an arbitrary time source, which is used to present
   a virtual time based on the instruction counter when icount mode is
   active.

 * QEMUTimer is associated with a QEMUClock.  This allows timers that
   only expire when the vm_clock is enabled, for example.

 * Modifying a QEMUTimer uses qemu_notify_event() since it may be called
   from a vcpu thread while the iothread is blocked.

The steps to achieving this:

1. Drop alarm timers from qemu-timer.c and calculate g_poll() timeout
   instead for the main loop.

2. Introduce a per-AioContext aio_ctx_clock that can be used with
   qemu_new_timer() to create a QEMUTimer that expires during
   aio_poll().

3. Calculate g_poll() timeout for aio_ctx_clock in aio_poll().

Stefan