[Qemu-devel] [RFC PATCH V2 0/5] icount: Implement delay algorithm between guest and host clocks

[Sebastian Tanase]

The icount option already implemented in QEMU allows the guest to run at a 
theoretical
frequency of 1/(2^N) GHz (N is the icount parameter). The goal of this patch is 
to have a
real guest frequency close to the one imposed by using the icount option.

The main idea behind the algorithm is that we compare the virtual monotonic 
clock and the
host monotonic clock. For big icounts (on our test machine, an i5 CPU @ 
3.10GHz, icounts
starting at 6) the guest clock will be ahead of the host clock. In this case, 
we try to
sleep QEMU for the difference between the 2 clocks. Therefore, the guest would 
have
executed for a period almost equally to the one imposed by icount. We should 
point out
that the algorithm works only for those icounts that allow the guest clock to 
be in front
of the host clock.

The first patch adds the 'align' parameter for icount and changes the icount 
option from
'-icount [N|auto]' to '-icount [icount=][N|auto][,align=on|off]'.

The second patch exports 'icount_time_shift' so that it can be used in places 
other than
cpus.c; we need it in cpu-exec.c for calculating for how long we want QEMU to 
sleep.

The third patch implements the algorithm used for calculating the delay we want 
to sleep.
It uses the number of instructions executed by the virtual cpu and also the 
icount_time_shift.
All the parts in cpu-exec.c are only compiled for soft-mmu architectures 
otherwise the code
will not compile for all architectures (linux-user and soft-mmu).

The forth patch updates the algorithm to take into account an initial offset 
between clocks.
The offset can be 0 (for example on ARM) or another value (typically it is 
almost equal
to the realtime clock) on other architectures (for example on x86).

The fifth and final patch prints to the console whenever the guest clock runs 
behind the host
clock. The fastest printing speed is every 2 seconds, and we only print if the 
align option
is enabled.

v1 -> v2

* Signal errors in configure_icount() as suggested by Paolo Bonzini (patch 1).
* Update qtest.c to take into account the new configure_icount() (patch 1).
* Surround delay code in cpu-exec.c with !defined(CONFIG_USER_ONLY) (patch 3).
* Clean up delay code in cpu-exec.c as suggested by Paolo Bonzini
  and Juan Quintela (patch 3).
* Take into account initial offset between real and virtual clocks (patch 4).

Sebastian Tanase (5):
  icount: Add 'align' and 'icount' options
  icount: Make icount_time_shift available everywhere
  cpu_exec: Add sleeping algorithm
  icount_warp: Take into account initial offset between clocks
  cpu_exec: Print to console if the guest is late

 cpu-exec.c                | 108 +++++++++++++++++++++++++++++++++++++++++++++-
 cpus.c                    |  38 ++++++++++++++--
 include/qapi/qmp/qerror.h |   9 ++++
 include/qemu-common.h     |   6 ++-
 qemu-options.hx           |  19 ++++++--
 qtest.c                   |  19 +++++++-
 vl.c                      |  43 +++++++++++++++---
 7 files changed, 225 insertions(+), 17 deletions(-)

-- 
2.0.0.rc2