[RFC qemu legoater/aspeed-3.1 0/4] Handle short timer periods

[Andrew Jeffery]

Hello,

We've had an issue for a while, since the introduction of 4451d3f59f2a
("clocksource/drivers/fttmr010: Fix set_next_event handler") in Linux, where
our qemu instances have a "sticky" behaviour. The VM becomes unresponsive at
unexpected times for unpredicable but often long periods of time.

This series, along with the associated patch to Linux's fttmr010 driver[0],
aims to resolve it.

[0] http://patchwork.ozlabs.org/patch/1023363/

The series an RFC series because it doesn't fix all the cases, just
demonstrates a potential way forward. The approach is to provide back-pressure
- to test if the reload value is set to a value that's smaller than some
experimentally determined value (in this case, 20us), and if so, configure a
period of at least 20us. The MAX() of the requested and minimum values is then
set in the reload register rather than the requested value, which can then be
read back by Linux. The fttmr010 driver, upon observing the greater value,
starts pushing back on the clock event subsystem, which then iteratively
determines a minimum acceptible event period before proceeding with the boot
process.

The implementation does not take care of the match register cases, but at the
moment they are unused by Linux on Aspeed SoCs. The match register case is not
taken care of because I'm not sure if this implementation is what we want to
use going forward, or whether we want to do something that's completely hidden
in the qemu model. However taking advantage of Linux's existing support for
determining minimum timer periods seemed like easy solution.

The stickiness turns out to be a consequence of Linux configuring a very short
timer period (1us, mostly due to the timerio_rng driver), which causes qemu to
spend a large chunk of its timeslice handling the host timer interrupt rather
than executing guest code.

Please critique!

Andrew

Andrew Jeffery (4):
  timer: aspeed: Fire interrupt on failure to meet deadline
  timer: aspeed: Status register contains reload for stopped timer
  timer: aspeed: Fix match calculations
  timer: aspeed: Provide back-pressure information for short periods

 hw/misc/aspeed_scu.c            |  6 ++++++
 hw/timer/aspeed_timer.c         | 37 ++++++++++++++++++++++++++-------
 include/hw/timer/aspeed_timer.h |  1 +
 3 files changed, 36 insertions(+), 8 deletions(-)

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2.19.1