[PATCH v3 1/3] powerpc: add barrier after writing kernel PTE

[Scott Wood]

There is no barrier between something like ioremap() writing to
a PTE, and returning the value to a caller that may then store the
pointer in a place that is visible to other CPUs.  Such callers
generally don't perform barriers of their own.

Even if callers of ioremap() and similar things did use barriers,
the most logical choise would be smp_wmb(), which is not
architecturally sufficient when BookE hardware tablewalk is used.  A
full sync is specified by the architecture.

For userspace mappings, OTOH, we generally already have an lwsync due
to locking, and if we occasionally take a spurious fault due to not
having a full sync with hardware tablewalk, it will not be fatal
because we will retry rather than oops.

Signed-off-by: Scott Wood <scottwood at freescale.com>
---
v3: Only add a sync for kernel mappings, and add lwsync to kernel
mappings even on targets that can't have BookE hardware tablewalk.
---
 arch/powerpc/mm/pgtable_32.c |  1 +
 arch/powerpc/mm/pgtable_64.c | 12 ++++++++++++
 2 files changed, 13 insertions(+)

diff --git a/arch/powerpc/mm/pgtable_32.c b/arch/powerpc/mm/pgtable_32.c
index 6c856fb..bc4806c 100644
--- a/arch/powerpc/mm/pgtable_32.c
+++ b/arch/powerpc/mm/pgtable_32.c
@@ -296,6 +296,7 @@ int map_page(unsigned long va, phys_addr_t pa, int flags)
 		set_pte_at(&init_mm, va, pg, pfn_pte(pa >> PAGE_SHIFT,
 						     __pgprot(flags)));
 	}
+	smp_wmb();
 	return err;
 }
 
diff --git a/arch/powerpc/mm/pgtable_64.c b/arch/powerpc/mm/pgtable_64.c
index 536eec72..de83a39 100644
--- a/arch/powerpc/mm/pgtable_64.c
+++ b/arch/powerpc/mm/pgtable_64.c
@@ -153,6 +153,18 @@ int map_kernel_page(unsigned long ea, unsigned long pa, int flags)
 		}
 #endif /* !CONFIG_PPC_MMU_NOHASH */
 	}
+
+#ifdef CONFIG_PPC_BOOK3E_64
+	/*
+	 * With hardware tablewalk, a sync is needed to ensure that
+	 * subsequent accesses see the PTE we just wrote.  Unlike userspace
+	 * mappings, we can't tolerate spurious faults, so make sure
+	 * the new PTE will be seen the first time.
+	 */
+	mb();
+#else
+	smp_wmb();
+#endif
 	return 0;
 }
 
-- 
1.8.1.2