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>

1 files changed, 18 insertions, 1 deletions

diff --git a/arch/powerpc/include/asm/book3s/64/radix.h b/arch/powerpc/include/asm/book3s/64/radix.h
index 01f6c2ca7ecd..9c567d243f61 100644
--- a/arch/powerpc/include/asm/book3s/64/radix.h
+++ b/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)