From: Peter Maydell <peter.maydell@linaro.org>
To: qemu-devel@nongnu.org
Cc: Richard Henderson <richard.henderson@linaro.org>
Subject: [PATCH for-6.1 0/2] accel/tcg: Fix hang when running in icount mode
Date: Sun, 25 Jul 2021 18:44:03 +0100 [thread overview]
Message-ID: <20210725174405.24568-1-peter.maydell@linaro.org> (raw)
This patchset fixes the intermittent hang seen when running a guest in
icount mode, as reported in
https://gitlab.com/qemu-project/qemu/-/issues/499 .
The underlying cause of the hang is that code in cpu_loop_exec_tb()
was using CF_COUNT_MASK as the maximum possible number of instructions
it would try to execute from a TB when it set the icount_decr.u16.low
field. This is wrong, because (a) that field can validly be set to any
unsigned 16-bit integer and (b) now that CF_COUNT_MASK has been
reduced to 511 in commit 78ff82bb1b67c0d7, it might be less than the
number of insns in the TB.
Patch one fixes cpu_loop_exec_tb() to use the actual maximum valid
value for icount_decr.u16.low, which is 0xffff. Patch two adjusts the
"should we ask for a TB with exactly this many insns in it?" condition
so that instead of testing "cpu->icount_extra == 0", which should be
always true if (insns_left > 0 && insns_left < tb->icount), we assert
it instead. This assertion would have caught the bug fixed in patch
one.
Tested using the same iterating loop test described in the bug report;
without the fix QEMU hangs within a handful of iterations. With the
fix it managed 175 successful iterations before I got bored and hit ^C.
thanks
-- PMM
Peter Maydell (2):
accel/tcg: Don't use CF_COUNT_MASK as the max value of
icount_decr.u16.low
accel/tcg: Remove unnecessary check on icount_extra in
cpu_loop_exec_tb()
accel/tcg/cpu-exec.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
--
2.20.1
next reply other threads:[~2021-07-25 17:45 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-25 17:44 Peter Maydell [this message]
2021-07-25 17:44 ` [PATCH for-6.1 1/2] accel/tcg: Don't use CF_COUNT_MASK as the max value of icount_decr.u16.low Peter Maydell
2021-07-26 9:17 ` Philippe Mathieu-Daudé
2021-07-25 17:44 ` [PATCH for-6.1 2/2] accel/tcg: Remove unnecessary check on icount_extra in cpu_loop_exec_tb() Peter Maydell
2021-07-25 17:45 ` Peter Maydell
2021-07-25 18:11 ` [PATCH for-6.1 0/2] accel/tcg: Fix hang when running in icount mode Richard Henderson
2021-07-26 16:54 ` Richard Henderson
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