Re: [PATCH] mips64el-softmmu: Enable MTTCG

["Jiaxun Yang" <jiaxun.yang@flygoat.com>]

在2024年6月4日六月 下午1:43，Alex Bennée写道：
[...]
>
> I'm a little uncomfortable about making the success of MTTCG depend on
> the user options passed to the kernel.

Yes, I agree, it sounds awkward.

>
> How come a non-icount run depends on how many instructions are being run
> on each core? Why is time not based on the wallclock time and by
> definition synced between all cores?

That's the problem of Guest Kernel, the synchronisation mechanism is poorly
written that relies on instruction latency too much. I have patch pending to
improve it at guest side, but we can't fix it in existing binary.

This clock, CP0_Counter, as per ISA spec, is independent between cores and
needs to be synchronized by software. We can emulate it as a monotonic clock
source and Linux should be happy with it, but that is violating ISA spec.

>
>> In non-MTTCG build, the execution is slow enough so kernel won't
>> observe back drifts.
>>
>> Test results:
>>
>> With clocksource=MIPS
>> ```
>>  ~/tmp/retry/retry.py -n 100 -c -- ./qemu-system-mips64el \
>>     -display none -vga none -serial mon:stdio \
>>     -machine malta -kernel ./vmlinux-4.7.0-rc1.I6400 \
>>     -cpu I6400 -smp 8 -vga std \
>>     -append "printk.time=0 clocksource=MIPS console=tty0 console=ttyS0 panic=-1" \
>>     --no-reboot
>>
>> 100, 0, PASS, 5.258126, 100, 100, -
>> Results summary:
>> 0: 100 times (100.00%), avg time 6.508 (55.53 varience/7.45 deviation)
>> Ran command 100 times, 100 passes
>> ```
>>
>> With linux-next:
>> ```
>>  ~/tmp/retry/retry.py -n 100 -c -- ./qemu-system-mips64el \
>>     -display none -vga none -serial mon:stdio \
>>     -machine malta -kernel ~/linux-next/vmlinux \
>>     -cpu I6400 -smp 8 -vga std \
>>     -append "printk.time=0 console=tty0 console=ttyS0 panic=-1" \
>>     --no-reboot
>>
>> 100, 0, PASS, 4.507921, 100, 100, -
>> Results summary:
>> 0: 100 times (100.00%), avg time 4.233 (0.04 varience/0.21 deviation)
>> Ran command 100 times, 100 passes
>> ```
>>
>> Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
>> ---
>> I'll leave the test case alone as it's already marked as
>> QEMU_TEST_FLAKY_TESTS
>
> We should ensure we have some testing enabled if we switch this on. Was
> the FLAKY test marked for MTTCG issues or something else?

Tracking history, I think it was because this issue is still popping up after
disabling MTTCG.

I'll try to upgrade kernel used in this test.

Thanks
- Jiaxun
>
>> ---
>>  configs/targets/mips64el-softmmu.mak | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/configs/targets/mips64el-softmmu.mak b/configs/targets/mips64el-softmmu.mak
>> index 8d9ab3ddc4b1..199b1d909a7d 100644
>> --- a/configs/targets/mips64el-softmmu.mak
>> +++ b/configs/targets/mips64el-softmmu.mak
>> @@ -1,3 +1,4 @@
>>  TARGET_ARCH=mips64
>>  TARGET_BASE_ARCH=mips
>> +TARGET_SUPPORTS_MTTCG=y
>>  TARGET_NEED_FDT=y
>>
>> ---
>> base-commit: 248f6f62df073a3b4158fd0093863ab885feabb5
>> change-id: 20240511-mips_mttcg-47a6b19074b3
>>
>> Best regards,
>
> -- 
> Alex Bennée
> Virtualisation Tech Lead @ Linaro

-- 
- Jiaxun