diff --git a/arch/powerpc/include/asm/hmi.h b/arch/powerpc/include/asm/hmi.h
new file mode 100644
index 0000000..88b4901
--- /dev/null
+++ b/arch/powerpc/include/asm/hmi.h
@@ -0,0 +1,45 @@
+/*
+ * Hypervisor Maintenance Interrupt header file.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.
+ *
+ * Copyright 2015 IBM Corporation
+ * Author: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
+ */
+
+#ifndef __ASM_PPC64_HMI_H__
+#define __ASM_PPC64_HMI_H__
+
+#ifdef CONFIG_PPC_BOOK3S_64
+
+#define	CORE_TB_RESYNC_REQ_BIT		63
+#define MAX_SUBCORE_PER_CORE		4
+
+/*
+ * sibling_subcore_state structure is used to co-ordinate all threads
+ * during HMI to avoid TB corruption. This structure is allocated once
+ * per each core and shared by all threads on that core.
+ */
+struct sibling_subcore_state {
+	unsigned long	flags;
+	u8		in_guest[MAX_SUBCORE_PER_CORE];
+};
+
+extern void wait_for_subcore_guest_exit(void);
+extern void wait_for_tb_resync(void);
+#else
+static inline void wait_for_subcore_guest_exit(void) { }
+static inline void wait_for_tb_resync(void) { }
+#endif
+#endif /* __ASM_PPC64_HMI_H__ */
diff --git a/arch/powerpc/include/asm/paca.h b/arch/powerpc/include/asm/paca.h
index 546540b..4b17bd0 100644
--- a/arch/powerpc/include/asm/paca.h
+++ b/arch/powerpc/include/asm/paca.h
@@ -25,6 +25,7 @@
 #ifdef CONFIG_KVM_BOOK3S_64_HANDLER
 #include <asm/kvm_book3s_asm.h>
 #endif
+#include <asm/hmi.h>
 
 register struct paca_struct *local_paca asm("r13");
 
@@ -181,6 +182,11 @@ struct paca_struct {
 	 */
 	u16 in_mce;
 	u8 hmi_event_available;		 /* HMI event is available */
+	/*
+	 * Bitmap for sibling subcore status. See kvm/book3s_hv_ras.c for
+	 * more details
+	 */
+	struct sibling_subcore_state *sibling_subcore_state;
 #endif
 
 	/* Stuff for accurate time accounting */
diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 2da380f..6972a23 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -41,7 +41,7 @@ obj-$(CONFIG_VDSO32)		+= vdso32/
 obj-$(CONFIG_HAVE_HW_BREAKPOINT)	+= hw_breakpoint.o
 obj-$(CONFIG_PPC_BOOK3S_64)	+= cpu_setup_ppc970.o cpu_setup_pa6t.o
 obj-$(CONFIG_PPC_BOOK3S_64)	+= cpu_setup_power.o
-obj-$(CONFIG_PPC_BOOK3S_64)	+= mce.o mce_power.o
+obj-$(CONFIG_PPC_BOOK3S_64)	+= mce.o mce_power.o hmi.o
 obj64-$(CONFIG_RELOCATABLE)	+= reloc_64.o
 obj-$(CONFIG_PPC_BOOK3E_64)	+= exceptions-64e.o idle_book3e.o
 obj-$(CONFIG_PPC64)		+= vdso64/
diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 7716ceb..0607ae9 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -690,6 +690,8 @@ _GLOBAL(__replay_interrupt)
 BEGIN_FTR_SECTION
 	cmpwi	r3,0xe80
 	beq	h_doorbell_common
+	cmpwi	r3,0xe60
+	beq	hmi_exception_common
 FTR_SECTION_ELSE
 	cmpwi	r3,0xa00
 	beq	doorbell_super_common
@@ -1261,7 +1263,7 @@ fwnmi_data_area:
 
 	.globl hmi_exception_early
 hmi_exception_early:
-	EXCEPTION_PROLOG_1(PACA_EXGEN, NOTEST, 0xe60)
+	EXCEPTION_PROLOG_1(PACA_EXGEN, KVMTEST, 0xe62)
 	mr	r10,r1			/* Save r1			*/
 	ld	r1,PACAEMERGSP(r13)	/* Use emergency stack		*/
 	subi	r1,r1,INT_FRAME_SIZE	/* alloc stack frame		*/
diff --git a/arch/powerpc/kernel/hmi.c b/arch/powerpc/kernel/hmi.c
new file mode 100644
index 0000000..e3f738e
--- /dev/null
+++ b/arch/powerpc/kernel/hmi.c
@@ -0,0 +1,56 @@
+/*
+ * Hypervisor Maintenance Interrupt (HMI) handling.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.
+ *
+ * Copyright 2015 IBM Corporation
+ * Author: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
+ */
+
+#undef DEBUG
+
+#include <linux/types.h>
+#include <linux/compiler.h>
+#include <asm/paca.h>
+#include <asm/hmi.h>
+
+void wait_for_subcore_guest_exit(void)
+{
+	int i;
+
+	/*
+	 * NULL bitmap pointer indicates that KVM module hasn't
+	 * been loaded yet and hence no guests are running.
+	 * If no KVM is in use, no need to co-ordinate among threads
+	 * as all of them will always be in host and no one is going
+	 * to modify TB other than the opal hmi handler.
+	 * Hence, just return from here.
+	 */
+	if (!local_paca->sibling_subcore_state)
+		return;
+
+	for (i = 0; i < MAX_SUBCORE_PER_CORE; i++)
+		while (local_paca->sibling_subcore_state->in_guest[i])
+			cpu_relax();
+}
+
+void wait_for_tb_resync(void)
+{
+	if (!local_paca->sibling_subcore_state)
+		return;
+
+	while (test_bit(CORE_TB_RESYNC_REQ_BIT,
+				&local_paca->sibling_subcore_state->flags))
+		cpu_relax();
+}
diff --git a/arch/powerpc/kernel/idle_power7.S b/arch/powerpc/kernel/idle_power7.S
index 470ceeb..bb51129 100644
--- a/arch/powerpc/kernel/idle_power7.S
+++ b/arch/powerpc/kernel/idle_power7.S
@@ -270,8 +270,9 @@ ALT_FTR_SECTION_END_NESTED_IFSET(CPU_FTR_ARCH_207S, 66);		\
 	ld	r2,PACATOC(r13);					\
 	ld	r1,PACAR1(r13);						\
 	std	r3,ORIG_GPR3(r1);	/* Save original r3 */		\
-	li	r0,OPAL_HANDLE_HMI;	/* Pass opal token argument*/	\
-	bl	opal_call_realmode;					\
+	li	r3,0;			/* NULL argument */		\
+	bl	hmi_exception_realmode;					\
+	nop;								\
 	ld	r3,ORIG_GPR3(r1);	/* Restore original r3 */	\
 20:	nop;
 
diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index 9229ba6..9ec95da 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -60,6 +60,7 @@
 #include <asm/switch_to.h>
 #include <asm/tm.h>
 #include <asm/debug.h>
+#include <asm/hmi.h>
 #include <sysdev/fsl_pci.h>
 
 #if defined(CONFIG_DEBUGGER) || defined(CONFIG_KEXEC)
@@ -307,9 +308,13 @@ long hmi_exception_realmode(struct pt_regs *regs)
 {
 	__this_cpu_inc(irq_stat.hmi_exceptions);
 
+	wait_for_subcore_guest_exit();
+
 	if (ppc_md.hmi_exception_early)
 		ppc_md.hmi_exception_early(regs);
 
+	wait_for_tb_resync();
+
 	return 0;
 }
 
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 84fb4fc..cfb4a42 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -51,6 +51,7 @@
 #include <asm/switch_to.h>
 #include <asm/smp.h>
 #include <asm/dbell.h>
+#include <asm/hmi.h>
 #include <linux/gfp.h>
 #include <linux/vmalloc.h>
 #include <linux/highmem.h>
@@ -3394,6 +3395,38 @@ static struct kvmppc_ops kvm_ops_hv = {
 	.hcall_implemented = kvmppc_hcall_impl_hv,
 };
 
+static int kvm_init_subcore_bitmap(void)
+{
+	int i, j;
+	int nr_cores = cpu_nr_cores();
+	struct sibling_subcore_state *sibling_subcore_state;
+
+	for (i = 0; i < nr_cores; i++) {
+		int first_cpu = i * threads_per_core;
+		int node = cpu_to_node(first_cpu);
+
+		/* Ignore if it is already allocated. */
+		if (paca[first_cpu].sibling_subcore_state)
+			continue;
+
+		sibling_subcore_state =
+			kmalloc_node(sizeof(struct sibling_subcore_state),
+							GFP_KERNEL, node);
+		if (!sibling_subcore_state)
+			return -ENOMEM;
+
+		memset(sibling_subcore_state, 0,
+				sizeof(struct sibling_subcore_state));
+
+		for (j = 0; j < threads_per_core; j++) {
+			int cpu = first_cpu + j;
+
+			paca[cpu].sibling_subcore_state = sibling_subcore_state;
+		}
+	}
+	return 0;
+}
+
 static int kvmppc_book3s_init_hv(void)
 {
 	int r;
@@ -3404,6 +3437,10 @@ static int kvmppc_book3s_init_hv(void)
 	if (r < 0)
 		return -ENODEV;
 
+	r = kvm_init_subcore_bitmap();
+	if (r)
+		return r;
+
 	kvm_ops_hv.owner = THIS_MODULE;
 	kvmppc_hv_ops = &kvm_ops_hv;
 
diff --git a/arch/powerpc/kvm/book3s_hv_ras.c b/arch/powerpc/kvm/book3s_hv_ras.c
index 93b5f5c..0fa70a9 100644
--- a/arch/powerpc/kvm/book3s_hv_ras.c
+++ b/arch/powerpc/kvm/book3s_hv_ras.c
@@ -13,6 +13,9 @@
 #include <linux/kernel.h>
 #include <asm/opal.h>
 #include <asm/mce.h>
+#include <asm/machdep.h>
+#include <asm/cputhreads.h>
+#include <asm/hmi.h>
 
 /* SRR1 bits for machine check on POWER7 */
 #define SRR1_MC_LDSTERR		(1ul << (63-42))
@@ -140,3 +143,176 @@ long kvmppc_realmode_machine_check(struct kvm_vcpu *vcpu)
 {
 	return kvmppc_realmode_mc_power7(vcpu);
 }
+
+/* Check if dynamic split is in force and return subcore size accordingly. */
+static inline int kvmppc_cur_subcore_size(void)
+{
+	if (local_paca->kvm_hstate.kvm_split_mode)
+		return local_paca->kvm_hstate.kvm_split_mode->subcore_size;
+
+	return threads_per_subcore;
+}
+
+void kvmppc_subcore_enter_guest(void)
+{
+	int thread_id, subcore_id;
+
+	thread_id = cpu_thread_in_core(local_paca->paca_index);
+	subcore_id = thread_id / kvmppc_cur_subcore_size();
+
+	local_paca->sibling_subcore_state->in_guest[subcore_id] = 1;
+}
+
+void kvmppc_subcore_exit_guest(void)
+{
+	int thread_id, subcore_id;
+
+	thread_id = cpu_thread_in_core(local_paca->paca_index);
+	subcore_id = thread_id / kvmppc_cur_subcore_size();
+
+	local_paca->sibling_subcore_state->in_guest[subcore_id] = 0;
+}
+
+static bool kvmppc_tb_resync_required(void)
+{
+	if (test_and_set_bit(CORE_TB_RESYNC_REQ_BIT,
+				&local_paca->sibling_subcore_state->flags))
+		return false;
+
+	return true;
+}
+
+static void kvmppc_tb_resync_done(void)
+{
+	clear_bit(CORE_TB_RESYNC_REQ_BIT,
+			&local_paca->sibling_subcore_state->flags);
+}
+
+/*
+ * kvmppc_realmode_hmi_handler() is called only by primary thread during
+ * guest exit path.
+ *
+ * There are multiple reasons why HMI could occur, one of them is
+ * Timebase (TB) error. If this HMI is due to TB error, then TB would
+ * have been in stopped state. The opal hmi handler Will fix it and
+ * restore the TB value with host timebase value. For HMI caused due
+ * to non-TB errors, opal hmi handler will not touch/restore TB register
+ * and hence there won't be any change in TB value.
+ *
+ * Since we are not sure about the cause of this HMI, we can't be sure
+ * about the content of TB register whether it holds guest or host timebase
+ * value. Hence the idea is to resync the TB on every HMI, so that we
+ * know about the exact state of the TB value. Resync TB call will
+ * restore TB to host timebase.
+ *
+ * Things to consider:
+ * - On TB error, HMI interrupt is reported on all the threads of the core
+ *   that has encountered TB error irrespective of split-core mode.
+ * - The very first thread on the core that get chance to fix TB error
+ *   would rsync the TB with local chipTOD value.
+ * - The resync TB is a core level action i.e. it will sync all the TBs
+ *   in that core independent of split-core mode. This means if we trigger
+ *   TB sync from a thread from one subcore, it would affect TB values of
+ *   sibling subcores of the same core.
+ *
+ * All threads need to co-ordinate before making opal hmi handler.
+ * All threads will use sibling_subcore_state->in_guest[] (shared by all
+ * threads in the core) in paca which holds information about whether
+ * sibling subcores are in Guest mode or host mode. The in_guest[] array
+ * is of size MAX_SUBCORE_PER_CORE=4, indexed using subcore id to set/unset
+ * subcore status. Only primary threads from each subcore is responsible
+ * to set/unset its designated array element while entering/exiting the
+ * guset.
+ *
+ * After invoking opal hmi handler call, one of the thread (of entire core)
+ * will need to resync the TB. Bit 63 from subcore state bitmap flags
+ * (sibling_subcore_state->flags) will be used to co-ordinate between
+ * primary threads to decide who takes up the responsibility.
+ *
+ * This is what we do:
+ * - Primary thread from each subcore tries to set resync required bit[63]
+ *   of paca->sibling_subcore_state->flags.
+ * - The first primary thread that is able to set the flag takes the
+ *   responsibility of TB resync. (Let us call it as thread leader)
+ * - All other threads which are in host will call
+ *   wait_for_subcore_guest_exit() and wait for in_guest[0-3] from
+ *   paca->sibling_subcore_state to get cleared.
+ * - All the primary thread will clear its subcore status from subcore
+ *   state in_guest[] array respectively.
+ * - Once all primary threads clear in_guest[0-3], all of them will invoke
+ *   opal hmi handler.
+ * - Now all threads will wait for TB resync to complete by invoking
+ *   wait_for_tb_resync() except the thread leader.
+ * - Thread leader will do a TB resync by invoking opal_resync_timebase()
+ *   call and the it will clear the resync required bit.
+ * - All other threads will now come out of resync wait loop and proceed
+ *   with individual execution.
+ * - On return of this function, primary thread will signal all
+ *   secondary threads to proceed.
+ * - All secondary threads will eventually call opal hmi handler on
+ *   their exit path.
+ */
+
+long kvmppc_realmode_hmi_handler(void)
+{
+	int ptid = local_paca->kvm_hstate.ptid;
+	bool resync_req;
+
+	/* This is only called on primary thread. */
+	BUG_ON(ptid != 0);
+	__this_cpu_inc(irq_stat.hmi_exceptions);
+
+	/*
+	 * By now primary thread has already completed guest->host
+	 * partition switch but haven't signaled secondaries yet.
+	 * All the secondary threads on this subcore is waiting
+	 * for primary thread to signal them to go ahead.
+	 *
+	 * For threads from subcore which isn't in guest, they all will
+	 * wait until all other subcores on this core exit the guest.
+	 *
+	 * Now set the resync required bit. If you are the first to
+	 * set this bit then kvmppc_tb_resync_required() function will
+	 * return true. For rest all other subcores
+	 * kvmppc_tb_resync_required() will return false.
+	 *
+	 * If resync_req == true, then this thread is responsible to
+	 * initiate TB resync after hmi handler has completed.
+	 * All other threads on this core will wait until this thread
+	 * clears the resync required bit flag.
+	 */
+	resync_req = kvmppc_tb_resync_required();
+
+	/* Reset the subcore status to indicate it has exited guest */
+	kvmppc_subcore_exit_guest();
+
+	/*
+	 * Wait for other subcores on this core to exit the guest.
+	 * All the primary threads and threads from subcore that are
+	 * not in guest will wait here until all subcores are out
+	 * of guest context.
+	 */
+	wait_for_subcore_guest_exit();
+
+	/*
+	 * At this point we are sure that primary threads from each
+	 * subcore on this core have completed guest->host partition
+	 * switch. Now it is safe to call HMI handler.
+	 */
+	if (ppc_md.hmi_exception_early)
+		ppc_md.hmi_exception_early(NULL);
+
+	/*
+	 * Check if this thread is responsible to resync TB.
+	 * All other threads will wait until this thread completes the
+	 * TB resync.
+	 */
+	if (resync_req) {
+		opal_resync_timebase();
+		/* Reset TB resync req bit */
+		kvmppc_tb_resync_done();
+	} else {
+		wait_for_tb_resync();
+	}
+	return 0;
+}
diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index e571ad2..0d246fc 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
@@ -29,6 +29,7 @@
 #include <asm/kvm_book3s_asm.h>
 #include <asm/book3s/64/mmu-hash.h>
 #include <asm/tm.h>
+#include <asm/opal.h>
 
 #define VCPU_GPRS_TM(reg) (((reg) * ULONG_SIZE) + VCPU_GPR_TM)
 
@@ -373,6 +374,18 @@ kvm_secondary_got_guest:
 	lwsync
 	std	r0, HSTATE_KVM_VCORE(r13)
 
+	/*
+	 * All secondaries exiting guest will fall through this path.
+	 * Before proceeding, just check for HMI interrupt and
+	 * invoke opal hmi handler. By now we are sure that the
+	 * primary thread on this core/subcore has already made partition
+	 * switch/TB resync and we are good to call opal hmi handler.
+	 */
+	cmpwi	r12, BOOK3S_INTERRUPT_HMI
+	bne	kvm_no_guest
+
+	li	r3,0			/* NULL argument */
+	bl	hmi_exception_realmode
 /*
  * At this point we have finished executing in the guest.
  * We need to wait for hwthread_req to become zero, since
@@ -428,6 +441,22 @@ kvm_no_guest:
  */
 kvm_unsplit_nap:
 	/*
+	 * When secondaries are napping in kvm_unsplit_nap() with
+	 * hwthread_req = 1, HMI goes ignored even though subcores are
+	 * already exited the guest. Hence HMI keeps waking up secondaries
+	 * from nap in a loop and secondaries always go back to nap since
+	 * no vcore is assigned to them. This makes impossible for primary
+	 * thread to get hold of secondary threads resulting into a soft
+	 * lockup in KVM path.
+	 *
+	 * Let us check if HMI is pending and handle it before we go to nap.
+	 */
+	cmpwi	r12, BOOK3S_INTERRUPT_HMI
+	bne	55f
+	li	r3, 0			/* NULL argument */
+	bl	hmi_exception_realmode
+55:
+	/*
 	 * Ensure that secondary doesn't nap when it has
 	 * its vcore pointer set.
 	 */
@@ -601,6 +630,11 @@ BEGIN_FTR_SECTION
 	mtspr	SPRN_DPDES, r8
 END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 
+	/* Mark the subcore state as inside guest */
+	bl	kvmppc_subcore_enter_guest
+	nop
+	ld	r5, HSTATE_KVM_VCORE(r13)
+	ld	r4, HSTATE_KVM_VCPU(r13)
 	li	r0,1
 	stb	r0,VCORE_IN_GUEST(r5)	/* signal secondaries to continue */
 
@@ -1683,6 +1717,23 @@ BEGIN_FTR_SECTION
 	mtspr	SPRN_DPDES, r8
 END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 
+	/* If HMI, call kvmppc_realmode_hmi_handler() */
+	cmpwi	r12, BOOK3S_INTERRUPT_HMI
+	bne	27f
+	bl	kvmppc_realmode_hmi_handler
+	nop
+	li	r12, BOOK3S_INTERRUPT_HMI
+	/*
+	 * At this point kvmppc_realmode_hmi_handler would have resync-ed
+	 * the TB. Hence it is not required to subtract guest timebase
+	 * offset from timebase. So, skip it.
+	 *
+	 * Also, do not call kvmppc_subcore_exit_guest() because it has
+	 * been invoked as part of kvmppc_realmode_hmi_handler().
+	 */
+	b	30f
+
+27:
 	/* Subtract timebase offset from timebase */
 	ld	r8,VCORE_TB_OFFSET(r5)
 	cmpdi	r8,0
@@ -1698,8 +1749,13 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 	addis	r8,r8,0x100		/* if so, increment upper 40 bits */
 	mtspr	SPRN_TBU40,r8
 
+17:	bl	kvmppc_subcore_exit_guest
+	nop
+30:	ld	r5,HSTATE_KVM_VCORE(r13)
+	ld	r4,VCORE_KVM(r5)	/* pointer to struct kvm */
+
 	/* Reset PCR */
-17:	ld	r0, VCORE_PCR(r5)
+	ld	r0, VCORE_PCR(r5)
 	cmpdi	r0, 0
 	beq	18f
 	li	r0, 0
@@ -2461,6 +2517,8 @@ BEGIN_FTR_SECTION
 	cmpwi	r6, 3			/* hypervisor doorbell? */
 	beq	3f
 END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
+	cmpwi	r6, 0xa			/* Hypervisor maintenance ? */
+	beq	4f
 	li	r3, 1			/* anything else, return 1 */
 0:	blr
 
@@ -2482,6 +2540,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 	li	r3, -1
 	blr
 
+	/* Woken up due to Hypervisor maintenance interrupt */
+4:	li	r12, BOOK3S_INTERRUPT_HMI
+	li	r3, 1
+	blr
+
 /*
  * Determine what sort of external interrupt is pending (if any).
  * Returns:
