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