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Subject[PATCH v2 13/18] powerpc: Add support for setting SPRN_TIDR
Date
We need the SPRN_TIDR to be set for use with fast thread-wakeup (core-
to-core wakeup) and also with CAPI.

Each thread in a process needs to have a unique id within the process.
But as explained below, for now, we assign globally unique thread ids
to all threads in the system.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Philippe Bergheaud <felix@linux.vnet.ibm.com>
Signed-off-by: Christophe Lombard <clombard@linux.vnet.ibm.com>
---
Changelog[v3]
- Merge changes with and address comments to Christophe's patch.
(i.e drop CONFIG_PPC_VAS; use CONFIG_PPC64; check CPU_ARCH_300
before setting TIDR). Defer following to separate patches:
- emulation parts of Christophe's patch,
- setting TIDR for tasks other than 'current'
- setting feature bit in AT_HWCAP2

Changelog[v2]
- Michael Ellerman: Use an interface to assign TIDR so it is
assigned to only threads that need it; move assignment to
restore_sprs(). Drop lint from rebase;
---
arch/powerpc/include/asm/processor.h | 1 +
arch/powerpc/include/asm/switch_to.h | 3 +
arch/powerpc/kernel/process.c | 122 +++++++++++++++++++++++++++++++++++
3 files changed, 126 insertions(+)

diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index fab7ff8..58cc212 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -329,6 +329,7 @@ struct thread_struct {
*/
int dscr_inherit;
unsigned long ppr; /* used to save/restore SMT priority */
+ unsigned long tidr;
#endif
#ifdef CONFIG_PPC_BOOK3S_64
unsigned long tar;
diff --git a/arch/powerpc/include/asm/switch_to.h b/arch/powerpc/include/asm/switch_to.h
index 17c8380..f5da32f 100644
--- a/arch/powerpc/include/asm/switch_to.h
+++ b/arch/powerpc/include/asm/switch_to.h
@@ -91,4 +91,7 @@ static inline void clear_task_ebb(struct task_struct *t)
#endif
}

+extern int set_thread_tidr(struct task_struct *t);
+extern void clear_thread_tidr(struct task_struct *t);
+
#endif /* _ASM_POWERPC_SWITCH_TO_H */
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 37ed60b..d861fcd 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1120,6 +1120,13 @@ static inline void restore_sprs(struct thread_struct *old_thread,
mtspr(SPRN_TAR, new_thread->tar);
}
#endif
+#ifdef CONFIG_PPC64
+ if (old_thread->tidr != new_thread->tidr) {
+ /* TIDR should be non-zero only with ISA3.0. */
+ WARN_ON_ONCE(!cpu_has_feature(CPU_FTR_ARCH_300));
+ mtspr(SPRN_TIDR, new_thread->tidr);
+ }
+#endif
}

#ifdef CONFIG_PPC_BOOK3S_64
@@ -1438,9 +1445,117 @@ void flush_thread(void)
#endif /* CONFIG_HAVE_HW_BREAKPOINT */
}

+#ifdef CONFIG_PPC64
+static DEFINE_SPINLOCK(vas_thread_id_lock);
+static DEFINE_IDA(vas_thread_ida);
+
+/*
+ * We need to assign a unique thread id to each thread in a process.
+ *
+ * This thread id, referred to as TIDR, and separate from the Linux's tgid,
+ * is intended to be used to direct an ASB_Notify from the hardware to the
+ * thread, when a suitable event occurs in the system.
+ *
+ * One such event is a "paste" instruction in the context of Fast Thread
+ * Wakeup (aka Core-to-core wake up in the Virtual Accelerator Switchboard
+ * (VAS) in POWER9.
+ *
+ * To get a unique TIDR per process we could simply reuse task_pid_nr() but
+ * the problem is that task_pid_nr() is not yet available copy_thread() is
+ * called. Fixing that would require changing more intrusive arch-neutral
+ * code in code path in copy_process()?.
+ *
+ * Further, to assign unique TIDRs within each process, we need an atomic
+ * field (or an IDR) in task_struct, which again intrudes into the arch-
+ * neutral code. So try to assign globally unique TIDRs for now.
+ *
+ * NOTE: TIDR 0 indicates that the thread does not need a TIDR value.
+ * For now, only threads that expect to be notified by the VAS
+ * hardware need a TIDR value and we assign values > 0 for those.
+ */
+#define MAX_THREAD_CONTEXT ((1 << 16) - 1)
+static int assign_thread_tidr(void)
+{
+ int index;
+ int err;
+
+again:
+ if (!ida_pre_get(&vas_thread_ida, GFP_KERNEL))
+ return -ENOMEM;
+
+ spin_lock(&vas_thread_id_lock);
+ err = ida_get_new_above(&vas_thread_ida, 1, &index);
+ spin_unlock(&vas_thread_id_lock);
+
+ if (err == -EAGAIN)
+ goto again;
+ else if (err)
+ return err;
+
+ if (index > MAX_THREAD_CONTEXT) {
+ spin_lock(&vas_thread_id_lock);
+ ida_remove(&vas_thread_ida, index);
+ spin_unlock(&vas_thread_id_lock);
+ return -ENOMEM;
+ }
+
+ return index;
+}
+
+static void free_thread_tidr(int id)
+{
+ spin_lock(&vas_thread_id_lock);
+ ida_remove(&vas_thread_ida, id);
+ spin_unlock(&vas_thread_id_lock);
+}
+
+/*
+ * Clear any TIDR value assigned to this thread.
+ */
+void clear_thread_tidr(struct task_struct *t)
+{
+ if (!t->thread.tidr)
+ return;
+
+ if (!cpu_has_feature(CPU_FTR_ARCH_300)) {
+ WARN_ON_ONCE(1);
+ return;
+ }
+
+ mtspr(SPRN_TIDR, 0);
+ free_thread_tidr(t->thread.tidr);
+ t->thread.tidr = 0;
+}
+
+/*
+ * Assign a unique TIDR (thread id) for task @t and set it in the thread
+ * structure. For now, we only support setting TIDR for 'current' task.
+ */
+int set_thread_tidr(struct task_struct *t)
+{
+ if (!cpu_has_feature(CPU_FTR_ARCH_300))
+ return -EINVAL;
+
+ if (t != current)
+ return -EINVAL;
+
+ t->thread.tidr = assign_thread_tidr();
+ if (t->thread.tidr < 0)
+ return t->thread.tidr;
+
+ mtspr(SPRN_TIDR, t->thread.tidr);
+
+ return 0;
+}
+
+#endif /* CONFIG_PPC64 */
+
void
release_thread(struct task_struct *t)
{
+#ifdef CONFIG_PPC64
+ clear_thread_tidr(t);
+#endif
}

/*
@@ -1466,6 +1581,10 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)

clear_task_ebb(dst);

+#ifdef CONFIG_PPC64
+ dst->thread.tidr = 0;
+#endif
+
return 0;
}

@@ -1576,6 +1695,9 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
#endif

setup_ksp_vsid(p, sp);
+#ifdef CONFIG_PPC64
+ p->thread.tidr = 0;
+#endif

#ifdef CONFIG_PPC64
if (cpu_has_feature(CPU_FTR_DSCR)) {
--
2.7.4
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