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

[Richard Henderson <richard.henderson@linaro.org>]

On 7/25/21 8:11 AM, Richard Henderson wrote:
> 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>

Queued for 6.1.

r~