Subject: Re: [PATCH] powerpc/8xx: fix regression introduced by cache coherency rewrite

From: Benjamin Herrenschmidt <[email-masked]>
Date: Fri, 25 Sep 2009 13:03:47 +1000

> I think there's more finishyness to 8xx than we thought. IE. That
> tlbil_va might have more reasons to be there than what the comment
> seems to advertize. Can you try to move it even higher up ? IE.
> Unconditionally at the beginning of set_pte_filter ?
> 
> Also, if that doesn't help, can you try putting one in
> set_access_flags_filter() just below ?

Ok, I got a refresher on the whole concept of "unpopulated TLB entries"
on 8xx, and that's damn scary. I think what mislead me initially is that
the comment around the workaround is simply not properly describing the
extent of the problem :-)

So I'm not going to make the 8xx TLB miss code sane, that's beyond what
I'm prepare to do with it, but I suspect that this should fix it (on top
of upstream). Let me know if that's enough or if we also need to put
one of these in ptep_set_access_flags().

Please let me know if that works for you.

Cheers,
Ben.

diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
index 5304093..7a8e676 100644
--- a/arch/powerpc/mm/pgtable.c
+++ b/arch/powerpc/mm/pgtable.c
@@ -170,6 +170,16 @@ struct page * maybe_pte_to_page(pte_t pte)
 
 static pte_t set_pte_filter(pte_t pte, unsigned long addr)
 {
+#ifdef CONFIG_8xx
+	/* 8xx has a weird concept of "unpopulated" entries. When we take
+	 * a TLB miss for a non-valid PTE, we insert such an entry which
+	 * causes a page fault the next time around. This entry must now
+	 * be kicked out or we'll just fault again
+	 */
+	/* 8xx doesn't care about PID, size or ind args */
+	_tlbil_va(addr, 0, 0, 0);
+#endif /* CONFIG_8xx */
+
 	pte = __pte(pte_val(pte) & ~_PAGE_HPTEFLAGS);
 	if (pte_looks_normal(pte) && !(cpu_has_feature(CPU_FTR_COHERENT_ICACHE) ||
 				       cpu_has_feature(CPU_FTR_NOEXECUTE))) {
@@ -177,17 +187,6 @@ static pte_t set_pte_filter(pte_t pte, unsigned long addr)
 		if (!pg)
 			return pte;
 		if (!test_bit(PG_arch_1, &pg->flags)) {
-#ifdef CONFIG_8xx
-			/* On 8xx, cache control instructions (particularly
-			 * "dcbst" from flush_dcache_icache) fault as write
-			 * operation if there is an unpopulated TLB entry
-			 * for the address in question. To workaround that,
-			 * we invalidate the TLB here, thus avoiding dcbst
-			 * misbehaviour.
-			 */
-			/* 8xx doesn't care about PID, size or ind args */
-			_tlbil_va(addr, 0, 0, 0);
-#endif /* CONFIG_8xx */
 			flush_dcache_icache_page(pg);
 			set_bit(PG_arch_1, &pg->flags);
 		}