powerpc: Introduce msgsnd/doorbell barrier primitives

POWER9 changes requirements and adds new instructions for
synchronization.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/dbell.h

@@ -44,6 +44,17 @@ static inline void _ppc_msgsnd(u32 msg)
 		__asm__ __volatile__ (PPC_MSGSNDP(%0) : : "r" (msg));
 }
 
+/* sync before sending message */
+static inline void ppc_msgsnd_sync(void)
+{
+	__asm__ __volatile__ ("sync" : : : "memory");
+}
+
+/* sync after taking message interrupt */
+static inline void ppc_msgsync(void)
+{
+}
+
 #else /* CONFIG_PPC_BOOK3S */
 
 #define PPC_DBELL_MSGTYPE		PPC_DBELL
@@ -53,6 +64,17 @@ static inline void _ppc_msgsnd(u32 msg)
 	__asm__ __volatile__ (PPC_MSGSND(%0) : : "r" (msg));
 }
 
+/* sync before sending message */
+static inline void ppc_msgsnd_sync(void)
+{
+	__asm__ __volatile__ ("sync" : : : "memory");
+}
+
+/* sync after taking message interrupt */
+static inline void ppc_msgsync(void)
+{
+}
+
 #endif /* CONFIG_PPC_BOOK3S */
 
 extern void doorbell_global_ipi(int cpu);

arch/powerpc/include/asm/smp.h

@@ -128,6 +128,7 @@ extern const char *smp_ipi_name[];
 extern void smp_muxed_ipi_message_pass(int cpu, int msg);
 extern void smp_muxed_ipi_set_message(int cpu, int msg);
 extern irqreturn_t smp_ipi_demux(void);
+extern irqreturn_t smp_ipi_demux_relaxed(void);
 
 void smp_init_pSeries(void);
 void smp_init_cell(void);

arch/powerpc/kernel/dbell.c

@@ -38,7 +38,7 @@ void doorbell_global_ipi(int cpu)
 
 	kvmppc_set_host_ipi(cpu, 1);
 	/* Order previous accesses vs. msgsnd, which is treated as a store */
-	mb();
+	ppc_msgsnd_sync();
 	ppc_msgsnd(PPC_DBELL_MSGTYPE, 0, tag);
 }
 
@@ -53,7 +53,7 @@ void doorbell_core_ipi(int cpu)
 
 	kvmppc_set_host_ipi(cpu, 1);
 	/* Order previous accesses vs. msgsnd, which is treated as a store */
-	mb();
+	ppc_msgsnd_sync();
 	ppc_msgsnd(PPC_DBELL_MSGTYPE, 0, tag);
 }
 
@@ -82,12 +82,14 @@ void doorbell_exception(struct pt_regs *regs)
 
 	irq_enter();
 
+	ppc_msgsync();
+
 	may_hard_irq_enable();
 
 	kvmppc_set_host_ipi(smp_processor_id(), 0);
 	__this_cpu_inc(irq_stat.doorbell_irqs);
 
-	smp_ipi_demux();
+	smp_ipi_demux_relaxed(); /* already performed the barrier */
 
 	irq_exit();
 	set_irq_regs(old_regs);

arch/powerpc/kernel/smp.c

@@ -245,12 +245,18 @@ void smp_muxed_ipi_message_pass(int cpu, int msg)
 #endif
 
 irqreturn_t smp_ipi_demux(void)
+{
+	mb();	/* order any irq clear */
+
+	return smp_ipi_demux_relaxed();
+}
+
+/* sync-free variant. Callers should ensure synchronization */
+irqreturn_t smp_ipi_demux_relaxed(void)
 {
 	struct cpu_messages *info;
 	unsigned long all;
 
-	mb();	/* order any irq clear */
-
 	info = this_cpu_ptr(&ipi_message);
 	do {
 		all = xchg(&info->messages, 0);