Re: [PATCH for-6.1 0/2] accel/tcg: Fix hang when running in icount mode

[Richard Henderson]

On 7/25/21 7:44 AM, Peter Maydell wrote:
> 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()

Nice one.
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~