powerpc/kvm: Fix lockups when running KVM guests on Power8

[Michael Ellerman]

On Thu, 2018-04-19 at 13:29:50 UTC, Michael Ellerman wrote:
> When running KVM guests on Power8 we can see a lockup where one CPU
> stops responding. This often leads to a message such as:
> 
>   watchdog: CPU 136 detected hard LOCKUP on other CPUs 72
>   Task dump for CPU 72:
>   qemu-system-ppc R  running task    10560 20917  20908 0x00040004
> 
> And then backtraces on other CPUs, such as:
> 
>   Task dump for CPU 48:
>   ksmd            R  running task    10032  1519      2 0x00000804
>   Call Trace:
>     ...
>     --- interrupt: 901 at smp_call_function_many+0x3c8/0x460
>         LR = smp_call_function_many+0x37c/0x460
>     pmdp_invalidate+0x100/0x1b0
>     __split_huge_pmd+0x52c/0xdb0
>     try_to_unmap_one+0x764/0x8b0
>     rmap_walk_anon+0x15c/0x370
>     try_to_unmap+0xb4/0x170
>     split_huge_page_to_list+0x148/0xa30
>     try_to_merge_one_page+0xc8/0x990
>     try_to_merge_with_ksm_page+0x74/0xf0
>     ksm_scan_thread+0x10ec/0x1ac0
>     kthread+0x160/0x1a0
>     ret_from_kernel_thread+0x5c/0x78
> 
> This is caused by commit 8c1c7fb0b5ec ("powerpc/64s/idle: avoid sync
> for KVM state when waking from idle"), which added a check in
> pnv_powersave_wakeup() to see if the kvm_hstate.hwthread_state is
> already set to KVM_HWTHREAD_IN_KERNEL, and if so to skip the store and
> test of kvm_hstate.hwthread_req.
> 
> The problem is that the primary does not set KVM_HWTHREAD_IN_KVM when
> entering the guest, so it can then come out to cede with
> KVM_HWTHREAD_IN_KERNEL set. It can then go idle in kvm_do_nap after
> setting hwthread_req to 1, but because hwthread_state is still
> KVM_HWTHREAD_IN_KERNEL we will skip the test of hwthread_req when we
> wake up from idle and won't go to kvm_start_guest. From there the
> thread will return somewhere garbage and crash.
> 
> Fix it by skipping the store of hwthread_state, but not the test of
> hwthread_req, when coming out of idle. It's OK to skip the sync in
> that case because hwthread_req will have been set on the same thread,
> so there is no synchronisation required.
> 
> Fixes: 8c1c7fb0b5ec ("powerpc/64s/idle: avoid sync for KVM state when waking from idle")
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>

Applied to powerpc fixes.

https://git.kernel.org/powerpc/c/56376c5864f8ff4ba7c78a80ae857e

cheers