Patch "powerpc/mm: Add tracking of the number of coprocessors using a context" has been added to the 4.15-stable tree

This is a note to let you know that I've just added the patch titled

powerpc/mm: Add tracking of the number of coprocessors using a context

to the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
powerpc-mm-add-tracking-of-the-number-of-coprocessors-using-a-context.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Benjamin Herrenschmidt <[email protected]>
Date: Fri, 23 Mar 2018 09:29:05 +1100
Subject: powerpc/mm: Add tracking of the number of coprocessors using a context

From: Benjamin Herrenschmidt <[email protected]>

commit aff6f8cb3e2170b9e58b0932bce7bfb492775e23 upstream.

Currently, when using coprocessors (which use the Nest MMU), we simply increment the active_cpu count to force all TLB invalidations to be come broadcast.

Unfortunately, due to an errata in POWER9, we will need to know more specifically that coprocessors are in use.

This maintains a separate copros counter in the MMU context for that purpose.

NB. The commit mentioned in the fixes tag below is not at fault for the bug we're fixing in this commit and the next, but this fix applies on top the infrastructure it introduced.

---
 arch/powerpc/include/asm/book3s/64/mmu.h |    3 +++
 arch/powerpc/include/asm/mmu_context.h   |   18 +++++++++++++-----
 arch/powerpc/mm/mmu_context_book3s64.c   |    1 +
 3 files changed, 17 insertions(+), 5 deletions(-)

@@ -87,6 +87,9 @@ typedef struct {
 	/* Number of bits in the mm_cpumask */
 	atomic_t active_cpus;
 
+	/* Number of users of the external (Nest) MMU */
+	atomic_t copros;
+
 	/* NPU NMMU context */
 	struct npu_context *npu_context;
 

@@ -92,15 +92,23 @@ static inline void dec_mm_active_cpus(st
 static inline void mm_context_add_copro(struct mm_struct *mm)
 {
 	/*
-	 * On hash, should only be called once over the lifetime of
-	 * the context, as we can't decrement the active cpus count
-	 * and flush properly for the time being.
+	 * If any copro is in use, increment the active CPU count
+	 * in order to force TLB invalidations to be global as to
+	 * propagate to the Nest MMU.
 	 */
-	inc_mm_active_cpus(mm);
+	if (atomic_inc_return(&mm->context.copros) == 1)
+		inc_mm_active_cpus(mm);
 }
 
 static inline void mm_context_remove_copro(struct mm_struct *mm)
 {
+	int c;
+
+	c = atomic_dec_if_positive(&mm->context.copros);
+
+	/* Detect imbalance between add and remove */
+	WARN_ON(c < 0);
+
 	/*
 	 * Need to broadcast a global flush of the full mm before
 	 * decrementing active_cpus count, as the next TLBI may be

@@ -111,7 +119,7 @@ static inline void mm_context_remove_cop
 	 * for the time being. Invalidations will remain global if
 	 * used on hash.
 	 */
-	if (radix_enabled()) {
+	if (c == 0 && radix_enabled()) {
 		flush_all_mm(mm);
 		dec_mm_active_cpus(mm);
 	}

@@ -171,6 +171,7 @@ int init_new_context(struct task_struct
 	mm_iommu_init(mm);
 #endif
 	atomic_set(&mm->context.active_cpus, 0);
+	atomic_set(&mm->context.copros, 0);
 
 	return 0;
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/powerpc-mm-add-tracking-of-the-number-of-coprocessors-using-a-context.patch queue-4.15/powerpc-mm-workaround-nest-mmu-bug-with-tlb-invalidations.patch