Re: [PATCH] powerpc: warn on emulation of dcbz instruction

Le 16/09/2021 à 09:16, Benjamin Herrenschmidt a écrit :

On Thu, 2021-09-16 at 17:15 +1000, Benjamin Herrenschmidt wrote:

quoted

On Wed, 2021-09-15 at 16:31 +0200, Christophe Leroy wrote:

quoted

dcbz instruction shouldn't be used on non-cached memory. Using
it on non-cached memory can result in alignment exception and
implies a heavy handling.

Instead of silentely emulating the instruction and resulting in
high
performance degradation, warn whenever an alignment exception is
taken due to dcbz, so that the user is made aware that dcbz
instruction has been used unexpectedly.

Reported-by: Stan Johnson <redacted>
Cc: Finn Thain <fthain@linux-m68k.org>
Signed-off-by: Christophe Leroy <redacted>
---
  arch/powerpc/kernel/align.c | 1 +
  1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/kernel/align.c

b/arch/powerpc/kernel/align.c
index bbb4181621dd..adc3a4a9c6e4 100644

--- a/arch/powerpc/kernel/align.c
+++ b/arch/powerpc/kernel/align.c

@@ -349,6 +349,7 @@ int fix_alignment(struct pt_regs *regs)
  		if (op.type != CACHEOP + DCBZ)
  			return -EINVAL;
  		PPC_WARN_ALIGNMENT(dcbz, regs);
+		WARN_ON_ONCE(1);

This is heavy handed ... It will be treated as an oops by various
things uselessly spit out a kernel backtrace. Isn't
PPC_WARN_ALIGNMENT
enough ?

PPC_WARN_ALIGNMENT() only warns if explicitely activated, I want to 
catch uses on 'dcbz' on non-cached memory all the time as they are most 
often the result of using memset() instead of memset_io().

Ah I saw your other one about fbdev...  Ok what about you do that in a
if (!user_mode(regs)) ?

Yes I can do WARN_ON_ONCE(!user_mode(regs)); instead.

Indeed the kernel should not do that.

Does userspace accesses non-cached memory directly ?

Christophe