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[Qemu-devel] TCG icount interaction with timer deadlines
From: |
Peter Maydell |
Subject: |
[Qemu-devel] TCG icount interaction with timer deadlines |
Date: |
Thu, 5 Apr 2018 17:01:32 +0100 |
Does anybody understand how icount TCG is supposed to arrange to
respect timer deadlines?
https://bugs.launchpad.net/qemu/+bug/1754038 has a test case which
shows that we don't get this right.
At the moment what happens is:
* when we're about to call tcg_cpu_exec(), we call
prepare_icount_for_run(), which looks at the closest clock deadline
to figure out how many insns to execute before dropping out, with
a cap at INT32_MAX nanoseconds
* however, if the guest reprograms the clock during the tcg_cpu_exec()
run, we don't do anything to cause us to stop earlier
* so we blithely continue on til that INT32_MAX cap, and then
belatedly notice that we should have fired a timer interrupt
In the test case this manifests as the first timer interrupt being
very delayed, because the first tcg_cpu_exec() goes from "start of
program" to INT32_MAX nanoseconds. Later interrupts happen OK, because
the guest isn't reprogramming the timer interrupt, so the deadline
picked at the start of each tcg_cpu_exec run is correct.
What should we be doing to arrange to stop execution of the
tcg_cpu_exec() earlier when the deadline moves closer?
thanks
-- PMM
- [Qemu-devel] TCG icount interaction with timer deadlines,
Peter Maydell <=