Hello,

on POWER8 KVM hosts lock up since commit 10d91611f426 ("powerpc/64s:
Reimplement book3s idle code in C").

The symptom is host locking up completely after some hours of KVM
workload with messages like

2020-08-30T10:51:31+00:00 obs-power8-01 kernel: KVM: couldn't grab cpu 47
2020-08-30T10:51:31+00:00 obs-power8-01 kernel: KVM: couldn't grab cpu 71
2020-08-30T10:51:31+00:00 obs-power8-01 kernel: KVM: couldn't grab cpu 47
2020-08-30T10:51:31+00:00 obs-power8-01 kernel: KVM: couldn't grab cpu 71
2020-08-30T10:51:31+00:00 obs-power8-01 kernel: KVM: couldn't grab cpu 47

printed before the host locks up.

The machines run sandboxed builds which is a mixed workload resulting in
IO/single core/mutiple core load over time and there are periods of no
activity and no VMS runnig as well. The VMs are shortlived so VM
setup/terdown is somewhat excercised as well.

POWER9 with the new guest entry fast path does not seem to be affected.

Reverted the patch and the followup idle fixes on top of 5.2.14 and
re-applied commit a3f3072db6ca ("powerpc/powernv/idle: Restore IAMR
after idle") which gives same idle code as 5.1.16 and the kernel seems
stable.

Config is attached.

I cannot easily revert this commit, especially if I want to use the same
kernel on POWER8 and POWER9 - many of the POWER9 fixes are applicable
only to the new idle code.

Any idea what can be the problem?

Thanks

Michal


commit 87706fefb43f4b9320fb5b18b408237c6200468a
Author: Russell Currey <ruscur@russell.cc>
Date:   Thu Apr 18 16:51:16 2019 +1000

    powerpc/powernv/idle: Restore IAMR after idle

    ...

commit b01a02e38c2a8d93e81133cb4ed5d92644453fbf
Author: Michal Suchanek <msuchanek@suse.de>
Date:   Sun Aug 30 12:14:36 2020 +0200

    Revert "powerpc/64s: Reimplement book3s idle code in C"
    
    This reverts commit 10d91611f426d4bafd2a83d966c36da811b2f7ad.

commit 2718c37d8e1e37f35e596727a7d39275a602f071
Author: Michal Suchanek <msuchanek@suse.de>
Date:   Sun Aug 30 12:10:17 2020 +0200

    Revert "powerpc/powernv/idle: Restore AMR/UAMOR/AMOR/IAMR after idle"
    
    This reverts commit e9cef0189c5b217fcd4788562862defc27632a01.

commit 7cf36babc860e5d2cd130dba1360b132a74f16d7
Author: Michal Suchanek <msuchanek@suse.de>
Date:   Sun Aug 30 12:09:23 2020 +0200

    Revert "powerpc/powernv/idle: Fix restore of SPRN_LDBAR for POWER9 stop state."
    
    This reverts commit d71acd3f404e65b4cfcbdd3ae096c15c847d7d04.