Re: [PATCH] mips64el-softmmu: Enable MTTCG

[Pierrick Bouvier <pierrick.bouvier@linaro.org>]

On 5/11/24 13:26, Jiaxun Yang wrote:
> MTTCG was disabled in a092a9554771 ("configure: disable MTTCG
> for MIPS guests") due to test case instability.
> 
> I was able to reproduce this issue with in latest QEMU and look
> into reason behind that.
> 
> What actually happend is kernel's CP0 timer synchronisation
> mechanism assumed a consistent latency in memory access between
> cores, which TCG can't guarantee. Thus there is a huge drift in
> count value between cores, and in early kernel versions CP0 timer
> is always used as sched_clock.
> 
> sched_clock drift back on some cores triggered RCU watchdog in
> some extreme cases.
> 
> This can be resolved by setting clocksource to MIPS, which allows
> clocksource to drift together with sched_clock. However this will
> leed to other problems after boot.
> 
> Another option would beupdating kernel to later version, which
> will use GIC as sched_clock.
> 
> 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
> ---
>   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,

Hi Jiaxun,
Thanks for your analysis!

We should see to update concerned test in another series.
I'm not sure which way is preferred between updating kernel used or 
changing current command line.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>