# git rev-parse -q --verify 56376c5864f8ff4ba7c78a80ae857eee3b1d23d8^{commit}
56376c5864f8ff4ba7c78a80ae857eee3b1d23d8
already have revision, skipping fetch
# git checkout -q -f -B kisskb 56376c5864f8ff4ba7c78a80ae857eee3b1d23d8
# git clean -qxdf
# < git log -1
# commit 56376c5864f8ff4ba7c78a80ae857eee3b1d23d8
# Author: Michael Ellerman <mpe@ellerman.id.au>
# Date:   Thu Apr 19 16:22:20 2018 +1000
# 
#     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@ellerman.id.au>
# < /opt/cross/kisskb/gcc-5.3.0-nolibc/powerpc64-linux/bin/powerpc64-linux-gcc --version
# < git log --format=%s --max-count=1 56376c5864f8ff4ba7c78a80ae857eee3b1d23d8
# < make -s -j 48 ARCH=powerpc O=/kisskb/build/powerpc-fixes_chrp32_defconfig_powerpc-5.3 CROSS_COMPILE=/opt/cross/kisskb/gcc-5.3.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  chrp32_defconfig
# make -s -j 48 ARCH=powerpc O=/kisskb/build/powerpc-fixes_chrp32_defconfig_powerpc-5.3 CROSS_COMPILE=/opt/cross/kisskb/gcc-5.3.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  
Completed OK
# rm -rf /kisskb/build/powerpc-fixes_chrp32_defconfig_powerpc-5.3
# Build took: 0:00:42.610342