powerpc/mm: Rework I$/D$ coherency (v3)

This patch reworks the way we do I and D cache coherency on PowerPC.

The "old" way was split in 3 different parts depending on the processor type:

   - Hash with per-page exec support (64-bit and >= POWER4 only) does it
at hashing time, by preventing exec on unclean pages and cleaning pages
on exec faults.

   - Everything without per-page exec support (32-bit hash, 8xx, and
64-bit < POWER4) does it for all page going to user space in update_mmu_cache().

   - Embedded with per-page exec support does it from do_page_fault() on
exec faults, in a way similar to what the hash code does.

That leads to confusion, and bugs. For example, the method using update_mmu_cache()
is racy on SMP where another processor can see the new PTE and hash it in before
we have cleaned the cache, and then blow trying to execute. This is hard to hit but
I think it has bitten us in the past.

Also, it's inefficient for embedded where we always end up having to do at least
one more page fault.

This reworks the whole thing by moving the cache sync into two main call sites,
though we keep different behaviours depending on the HW capability. The call
sites are set_pte_at() which is now made out of line, and ptep_set_access_flags()
which joins the former in pgtable.c

The base idea for Embedded with per-page exec support, is that we now do the
flush at set_pte_at() time when coming from an exec fault, which allows us
to avoid the double fault problem completely (we can even improve the situation
more by implementing TLB preload in update_mmu_cache() but that's for later).

If for some reason we didn't do it there and we try to execute, we'll hit
the page fault, which will do a minor fault, which will hit ptep_set_access_flags()
to do things like update _PAGE_ACCESSED or _PAGE_DIRTY if needed, we just make
this guys also perform the I/D cache sync for exec faults now. This second path
is the catch all for things that weren't cleaned at set_pte_at() time.

For cpus without per-pag exec support, we always do the sync at set_pte_at(),
thus guaranteeing that when the PTE is visible to other processors, the cache
is clean.

For the 64-bit hash with per-page exec support case, we keep the old mechanism
for now. I'll look into changing it later, once I've reworked a bit how we
use _PAGE_EXEC.

This is also a first step for adding _PAGE_EXEC support for embedded platforms

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 84 insertions, 0 deletions

diff --git a/arch/powerpc/include/asm/pgtable.h b/arch/powerpc/include/asm/pgtable.h
index 07f55e601696..5c1c4880723c 100644
--- a/arch/powerpc/include/asm/pgtable.h
+++ b/arch/powerpc/include/asm/pgtable.h
@@ -6,7 +6,17 @@
 #include <asm/processor.h>              /* For TASK_SIZE */
 #include <asm/mmu.h>
 #include <asm/page.h>
+
 struct mm_struct;
+
+#ifdef CONFIG_DEBUG_VM
+extern void assert_pte_locked(struct mm_struct *mm, unsigned long addr);
+#else /* CONFIG_DEBUG_VM */
+static inline void assert_pte_locked(struct mm_struct *mm, unsigned long addr)
+{
+}
+#endif /* !CONFIG_DEBUG_VM */
+
 #endif /* !__ASSEMBLY__ */
 
 #if defined(CONFIG_PPC64)
@@ -17,6 +27,80 @@ struct mm_struct;
 
 #ifndef __ASSEMBLY__
 
+/* Insert a PTE, top-level function is out of line. It uses an inline
+ * low level function in the respective pgtable-* files
+ */
+extern void set_pte_at(struct mm_struct *mm, unsigned long addr, pte_t *ptep,
+                       pte_t pte);
+
+/* This low level function performs the actual PTE insertion
+ * Setting the PTE depends on the MMU type and other factors. It's
+ * an horrible mess that I'm not going to try to clean up now but
+ * I'm keeping it in one place rather than spread around
+ */
+static inline void __set_pte_at(struct mm_struct *mm, unsigned long addr,
+                                pte_t *ptep, pte_t pte, int percpu)
+{
+#if defined(CONFIG_PPC_STD_MMU_32) && defined(CONFIG_SMP) && !defined(CONFIG_PTE_64BIT)
+        /* First case is 32-bit Hash MMU in SMP mode with 32-bit PTEs. We use the
+         * helper pte_update() which does an atomic update. We need to do that
+         * because a concurrent invalidation can clear _PAGE_HASHPTE. If it's a
+         * per-CPU PTE such as a kmap_atomic, we do a simple update preserving
+         * the hash bits instead (ie, same as the non-SMP case)
+         */
+        if (percpu)
+                *ptep = __pte((pte_val(*ptep) & _PAGE_HASHPTE)
+                              | (pte_val(pte) & ~_PAGE_HASHPTE));
+        else
+                pte_update(ptep, ~_PAGE_HASHPTE, pte_val(pte));
+
+#elif defined(CONFIG_PPC32) && defined(CONFIG_PTE_64BIT) && defined(CONFIG_SMP)
+        /* Second case is 32-bit with 64-bit PTE in SMP mode. In this case, we
+         * can just store as long as we do the two halves in the right order
+         * with a barrier in between. This is possible because we take care,
+         * in the hash code, to pre-invalidate if the PTE was already hashed,
+         * which synchronizes us with any concurrent invalidation.
+         * In the percpu case, we also fallback to the simple update preserving
+         * the hash bits
+         */
+        if (percpu) {
+                *ptep = __pte((pte_val(*ptep) & _PAGE_HASHPTE)
+                              | (pte_val(pte) & ~_PAGE_HASHPTE));
+                return;
+        }
+#if _PAGE_HASHPTE != 0
+        if (pte_val(*ptep) & _PAGE_HASHPTE)
+                flush_hash_entry(mm, ptep, addr);
+#endif
+        __asm__ __volatile__("\
+                stw%U0%X0 %2,%0\n\
+                eieio\n\
+                stw%U0%X0 %L2,%1"
+        : "=m" (*ptep), "=m" (*((unsigned char *)ptep+4))
+        : "r" (pte) : "memory");
+
+#elif defined(CONFIG_PPC_STD_MMU_32)
+        /* Third case is 32-bit hash table in UP mode, we need to preserve
+         * the _PAGE_HASHPTE bit since we may not have invalidated the previous
+         * translation in the hash yet (done in a subsequent flush_tlb_xxx())
+         * and see we need to keep track that this PTE needs invalidating
+         */
+        *ptep = __pte((pte_val(*ptep) & _PAGE_HASHPTE)
+                      | (pte_val(pte) & ~_PAGE_HASHPTE));
+
+#else
+        /* Anything else just stores the PTE normally. That covers all 64-bit
+         * cases, and 32-bit non-hash with 64-bit PTEs in UP mode
+         */
+        *ptep = pte;
+#endif
+}
+
+
+#define __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
+extern int ptep_set_access_flags(struct vm_area_struct *vma, unsigned long address,
+                                 pte_t *ptep, pte_t entry, int dirty);
+
 /*
  * Macro to mark a page protection value as "uncacheable".
  */