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

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Message-ID: <43f736d4-8625-2848-786f-79b902d5c753@csgroup.eu>
Date:   Thu, 16 Sep 2021 09:23:57 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Stan Johnson <userm57@...oo.com>,
        Finn Thain <fthain@...ux-m68k.org>
Subject: 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:
>> On Wed, 2021-09-15 at 16:31 +0200, Christophe Leroy wrote:
>>> 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 <userm57@...oo.com>
>>> Cc: Finn Thain <fthain@...ux-m68k.org>
>>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>>> ---
>>>   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