powerpc/64s/radix: avoid ptesync after set_pte and ptep_set_access_flags

The ISA suggests ptesync after setting a pte, to prevent a table walk
initiated by a subsequent access from missing that store and causing a
spurious fault. This is an architectual allowance that allows an
implementation's page table walker to be incoherent with the store
queue.

However there is no correctness problem in taking a spurious fault in
userspace -- the kernel copes with these at any time, so the updated
pte will be found eventually. Spurious kernel faults on vmap memory
must be avoided, so a ptesync is put into flush_cache_vmap.

On POWER9 so far I have not found a measurable window where this can
result in more minor faults, so as an optimisation, remove the costly
ptesync from pte updates. If an implementation benefits from ptesync,
it would be better to add it back in update_mmu_cache, so it's not
done for things like fork(2).

fork --fork --exec benchmark improved 5.2% (12400->13100).
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/book3s/64/radix.h

@@ -202,7 +202,24 @@ static inline void radix__set_pte_at(struct mm_struct *mm, unsigned long addr,
 				 pte_t *ptep, pte_t pte, int percpu)
 {
 	*ptep = pte;
-	asm volatile("ptesync" : : : "memory");
+
+	/*
+	 * The architecture suggests a ptesync after setting the pte, which
+	 * orders the store that updates the pte with subsequent page table
+	 * walk accesses which may load the pte. Without this it may be
+	 * possible for a subsequent access to result in spurious fault.
+	 *
+	 * This is not necessary for correctness, because a spurious fault
+	 * is tolerated by the page fault handler, and this store will
+	 * eventually be seen. In testing, there was no noticable increase
+	 * in user faults on POWER9. Avoiding ptesync here is a significant
+	 * win for things like fork. If a future microarchitecture benefits
+	 * from ptesync, it should probably go into update_mmu_cache, rather
+	 * than set_pte_at (which is used to set ptes unrelated to faults).
+	 *
+	 * Spurious faults to vmalloc region are not tolerated, so there is
+	 * a ptesync in flush_cache_vmap.
+	 */
 }
 
 static inline int radix__pmd_bad(pmd_t pmd)

arch/powerpc/include/asm/cacheflush.h

@@ -26,6 +26,19 @@
 #define flush_cache_vmap(start, end)		do { } while (0)
 #define flush_cache_vunmap(start, end)		do { } while (0)
 
+#ifdef CONFIG_BOOK3S_64
+/*
+ * Book3s has no ptesync after setting a pte, so without this ptesync it's
+ * possible for a kernel virtual mapping access to return a spurious fault
+ * if it's accessed right after the pte is set. The page fault handler does
+ * not expect this type of fault. flush_cache_vmap is not exactly the right
+ * place to put this, but it seems to work well enough.
+ */
+#define flush_cache_vmap(start, end)		do { asm volatile("ptesync"); } while (0)
+#else
+#define flush_cache_vmap(start, end)		do { } while (0)
+#endif
+
 #define ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE 1
 extern void flush_dcache_page(struct page *page);
 #define flush_dcache_mmap_lock(mapping)		do { } while (0)

arch/powerpc/mm/pgtable-radix.c

@@ -1115,5 +1115,5 @@ void radix__ptep_set_access_flags(struct vm_area_struct *vma, pte_t *ptep,
 		 * an access fault, which is defined by the architectue.
 		 */
 	}
-	asm volatile("ptesync" : : : "memory");
+	/* See ptesync comment in radix__set_pte_at */
 }