Re: [PATCH] mips64el-softmmu: Enable MTTCG

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

+Alex, Pierrick & Anton

On 11/5/24 22:26, 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,