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, 131 insertions, 0 deletions

diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
index 6d94116fdea1..a27ded3adac5 100644
--- a/arch/powerpc/mm/pgtable.c
+++ b/arch/powerpc/mm/pgtable.c
@@ -1,5 +1,6 @@
 /*
  * This file contains common routines for dealing with free of page tables
+ * Along with common page table handling code
  *
  *  Derived from arch/powerpc/mm/tlb_64.c:
  *    Copyright (C) 1995-1996 Gary Thomas (gdt@linuxppc.org)
@@ -115,3 +116,133 @@ void pte_free_finish(void)
 	pte_free_submit(*batchp);
 	*batchp = NULL;
 }
+
+/*
+ * Handle i/d cache flushing, called from set_pte_at() or ptep_set_access_flags()
+ */
+static pte_t do_dcache_icache_coherency(pte_t pte)
+{
+	unsigned long pfn = pte_pfn(pte);
+	struct page *page;
+
+	if (unlikely(!pfn_valid(pfn)))
+		return pte;
+	page = pfn_to_page(pfn);
+
+	if (!PageReserved(page) && !test_bit(PG_arch_1, &page->flags)) {
+		pr_debug("do_dcache_icache_coherency... flushing\n");
+		flush_dcache_icache_page(page);
+		set_bit(PG_arch_1, &page->flags);
+	}
+	else
+		pr_debug("do_dcache_icache_coherency... already clean\n");
+	return __pte(pte_val(pte) | _PAGE_HWEXEC);
+}
+
+static inline int is_exec_fault(void)
+{
+	return current->thread.regs && TRAP(current->thread.regs) == 0x400;
+}
+
+/* We only try to do i/d cache coherency on stuff that looks like
+ * reasonably "normal" PTEs. We currently require a PTE to be present
+ * and we avoid _PAGE_SPECIAL and _PAGE_NO_CACHE
+ */
+static inline int pte_looks_normal(pte_t pte)
+{
+	return (pte_val(pte) &
+		(_PAGE_PRESENT | _PAGE_SPECIAL | _PAGE_NO_CACHE)) ==
+		(_PAGE_PRESENT);
+}
+
+#if defined(CONFIG_PPC_STD_MMU)
+/* Server-style MMU handles coherency when hashing if HW exec permission
+ * is supposed per page (currently 64-bit only). Else, we always flush
+ * valid PTEs in set_pte.
+ */
+static inline int pte_need_exec_flush(pte_t pte, int set_pte)
+{
+	return set_pte && pte_looks_normal(pte) &&
+		!(cpu_has_feature(CPU_FTR_COHERENT_ICACHE) ||
+		  cpu_has_feature(CPU_FTR_NOEXECUTE));
+}
+#elif _PAGE_HWEXEC == 0
+/* Embedded type MMU without HW exec support (8xx only so far), we flush
+ * the cache for any present PTE
+ */
+static inline int pte_need_exec_flush(pte_t pte, int set_pte)
+{
+	return set_pte && pte_looks_normal(pte);
+}
+#else
+/* Other embedded CPUs with HW exec support per-page, we flush on exec
+ * fault if HWEXEC is not set
+ */
+static inline int pte_need_exec_flush(pte_t pte, int set_pte)
+{
+	return pte_looks_normal(pte) && is_exec_fault() &&
+		!(pte_val(pte) & _PAGE_HWEXEC);
+}
+#endif
+
+/*
+ * set_pte stores a linux PTE into the linux page table.
+ */
+void set_pte_at(struct mm_struct *mm, unsigned long addr, pte_t *ptep, pte_t pte)
+{
+#ifdef CONFIG_DEBUG_VM
+	WARN_ON(pte_present(*ptep));
+#endif
+	/* Note: mm->context.id might not yet have been assigned as
+	 * this context might not have been activated yet when this
+	 * is called.
+	 */
+	pte = __pte(pte_val(pte) & ~_PAGE_HPTEFLAGS);
+	if (pte_need_exec_flush(pte, 1))
+		pte = do_dcache_icache_coherency(pte);
+
+	/* Perform the setting of the PTE */
+	__set_pte_at(mm, addr, ptep, pte, 0);
+}
+
+/*
+ * This is called when relaxing access to a PTE. It's also called in the page
+ * fault path when we don't hit any of the major fault cases, ie, a minor
+ * update of _PAGE_ACCESSED, _PAGE_DIRTY, etc... The generic code will have
+ * handled those two for us, we additionally deal with missing execute
+ * permission here on some processors
+ */
+int ptep_set_access_flags(struct vm_area_struct *vma, unsigned long address,
+			  pte_t *ptep, pte_t entry, int dirty)
+{
+	int changed;
+	if (!dirty && pte_need_exec_flush(entry, 0))
+		entry = do_dcache_icache_coherency(entry);
+	changed = !pte_same(*(ptep), entry);
+	if (changed) {
+		assert_pte_locked(vma->vm_mm, address);
+		__ptep_set_access_flags(ptep, entry);
+		flush_tlb_page_nohash(vma, address);
+	}
+	return changed;
+}
+
+#ifdef CONFIG_DEBUG_VM
+void assert_pte_locked(struct mm_struct *mm, unsigned long addr)
+{
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd;
+
+	if (mm == &init_mm)
+		return;
+	pgd = mm->pgd + pgd_index(addr);
+	BUG_ON(pgd_none(*pgd));
+	pud = pud_offset(pgd, addr);
+	BUG_ON(pud_none(*pud));
+	pmd = pmd_offset(pud, addr);
+	BUG_ON(!pmd_present(*pmd));
+	BUG_ON(!spin_is_locked(pte_lockptr(mm, pmd)));
+}
+#endif /* CONFIG_DEBUG_VM */
+