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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?

-- PMM

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