I found
https://github.com/oxidecomputer/qemu-systick-bug are simple enough.
> thanks
> -- PMM
>
> On Thu, 15 Oct 2020 at 16:18, Peter Maydell <
peter.maydell@linaro.org> wrote:
> >
> > This patch series rewrites our implementation of the armv7m systick
> > timer to use ptimers.
> >
> > The armv7m systick timer is a 24-bit decrementing, wrap-on-zero,
> > clear-on-write counter. Our current implementation has various bugs
> > and dubious workarounds in it (for instance see
> >
https://bugs.launchpad.net/qemu/+bug/1872237).
> >
> > We have an implementation of a simple decrementing counter and we put
> > a lot of effort into making sure it handles the interesting corner
> > cases (like "spend a cycle at 0 before reloading"), so rather than
> > trying to fix these all over again in systick's hand-rolled countdown
> > code it's much simpler to just rewrite it to use a ptimer.
> >
> > Unfortunately this is a migration compatibility break, which will
> > affect all M-profile boards.
> >
> > Among other bugs, this fixes
> >
https://bugs.launchpad.net/qemu/+bug/1872237 : now writes to SYST_CVR
> > when the timer is enabled correctly do nothing; when the timer is
> > enabled via SYST_CSR.ENABLE, the ptimer code will (because of
> > POLICY_NO_IMMEDIATE_RELOAD) arrange that after one timer tick the
> > counter is reloaded from SYST_RVR and then counts down from there, as
> > the architecture requires.
> >
> > Side note: the trace from the test program in LP1872237 won't look
> > quite like it does on the hardware: under QEMU the "waiting for 1000
> > ms" debug printing generally reports a SYST_CVR value of 0. This is
> > because QEMU's emulated CPU is comparatively fast and our systick has a
> > hard-wired value of 1MHz for the frequency of the 'external reference
> > clock', which means that execution of the guest code reaches "read
> > SYST_CVR" before the first tick of the timer clock after enabling of
> > the timer (which is where the reload of SYST_CVR from SYST_RVR is
> > required). The exception is the first iteration, where the time QEMU
> > takes to translate the guest code is enough that the timer tick
> > happens before the register read. You can also get the timer tick to
> > win the race by fiddling around with the -icount option (which
> > effectively is slowing down the emulated CPU speed).
> >
> > Some day we should model both the 'system_clock_scale' (ie the CPU
> > clock frequency) and the 'external reference clock' as QEMU clock
> > source/sinks so that board code can specify the correct reference
> > clock frequency.
> >
> > Patch 1 is a minor tweak to the ptimer code to suppress a spurious
> > warning message for the "timer callback function disabled the ptimer"
> > case, which the systick timer uses. Patch 2 is the actual
> > conversion.
> >
> > thanks
> > -- PMM
> >
> >
> > Peter Maydell (2):
> > hw/core/ptimer: Support ptimer being disabled by timer callback
> > hw/timer/armv7m_systick: Rewrite to use ptimers
> >
> > include/hw/timer/armv7m_systick.h | 3 +-
> > hw/core/ptimer.c | 4 +
> > hw/timer/armv7m_systick.c | 124 +++++++++++++-----------------
> > 3 files changed, 58 insertions(+), 73 deletions(-)
> >
> > --
> > 2.20.1
> >
>
--
此致
礼
罗勇刚
Yours
sincerely,
Yonggang Luo