From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ozlabs.org (bilbo.ozlabs.org [203.11.71.1]) (using TLSv1.2 with cipher ADH-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 40Rfwy0bs8zF21H for ; Thu, 19 Apr 2018 23:29:54 +1000 (AEST) From: Michael Ellerman To: linuxppc-dev@ozlabs.org Cc: npiggin@gmail.com, paulus@samba.org Subject: [PATCH] powerpc/kvm: Fix lockups when running KVM guests on Power8 Date: Thu, 19 Apr 2018 23:29:50 +1000 Message-Id: <20180419132950.16752-1-mpe@ellerman.id.au> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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 --- arch/powerpc/kernel/idle_book3s.S | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/arch/powerpc/kernel/idle_book3s.S b/arch/powerpc/kernel/idle_book3s.S index 79d005445c6c..e734f6e45abc 100644 --- a/arch/powerpc/kernel/idle_book3s.S +++ b/arch/powerpc/kernel/idle_book3s.S @@ -553,12 +553,12 @@ ALT_FTR_SECTION_END_IFSET(CPU_FTR_ARCH_300) #ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE lbz r0,HSTATE_HWTHREAD_STATE(r13) cmpwi r0,KVM_HWTHREAD_IN_KERNEL - beq 1f + beq 0f li r0,KVM_HWTHREAD_IN_KERNEL stb r0,HSTATE_HWTHREAD_STATE(r13) /* Order setting hwthread_state vs. testing hwthread_req */ sync - lbz r0,HSTATE_HWTHREAD_REQ(r13) +0: lbz r0,HSTATE_HWTHREAD_REQ(r13) cmpwi r0,0 beq 1f b kvm_start_guest -- 2.14.1