Benjamin Herrenschmidt [off-list ref] wrote on 09/12/2009 05:19:59:

From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Joakim Tjernlund <redacted>
Cc: Scott Wood <redacted>, "linuxppc-dev@ozlabs.org" <linuxppc-
dev@ozlabs.org>, Rex Feany [off-list ref]
Date: 09/12/2009 05:20
Subject: Re: [PATCH 04/10] 8xx: Always pin kernel instruction TLB

On Fri, 2009-11-20 at 11:21 +0100, Joakim Tjernlund wrote:

quoted

Various kernel asm modifies SRR0/SRR1 just before executing
a rfi. If such code crosses a page boundary you risk a TLB miss
which will clobber SRR0/SRR1. Avoid this by always pinning
kernel instruction TLB space.

Signed-off-by: Joakim Tjernlund <redacted>
---
 arch/powerpc/kernel/head_8xx.S |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/powerpc/kernel/head_8xx.S b/arch/powerpc/kernel/head_8xx.S
index a9f1ace..e70503d 100644
--- a/arch/powerpc/kernel/head_8xx.S
+++ b/arch/powerpc/kernel/head_8xx.S

@@ -705,7 +705,7 @@ start_here:
  */
 initial_mmu:
    tlbia         /* Invalidate all TLB entries */
-#ifdef CONFIG_PIN_TLB
+#if 1 /* CONFIG_PIN_TLB */
    lis   r8, MI_RSV4I@h
    ori   r8, r8, 0x1c00
 #else

Not nice. Either remove the config option or make sure all those code
sequences are appropriately aligned so it doesn't happen. I recommend
the later :-)

The later isn't as simple :) I believe the bulk of such code in entry_32.S.
Anyhow, the config option is still valid as if enabled
it will pin several DTLB's too. Scott had some concerns about removing the
config option completely so this was the next best thing.

I'll apply the other patches.

OK, great.