From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
Stan Johnson <userm57@yahoo.com>,
Finn Thain <fthain@linux-m68k.org>
Subject: Re: [PATCH] powerpc: warn on emulation of dcbz instruction
Date: Thu, 16 Sep 2021 09:23:57 +0200 [thread overview]
Message-ID: <43f736d4-8625-2848-786f-79b902d5c753@csgroup.eu> (raw)
In-Reply-To: <eb1a39368401bf46e805ca64256604cc649f771e.camel@kernel.crashing.org>
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@yahoo.com>
>>> Cc: Finn Thain <fthain@linux-m68k.org>
>>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.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
next prev parent reply other threads:[~2021-09-16 7:24 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-15 14:31 [PATCH] powerpc: warn on emulation of dcbz instruction Christophe Leroy
2021-09-16 7:15 ` Benjamin Herrenschmidt
2021-09-16 7:16 ` Benjamin Herrenschmidt
2021-09-16 7:23 ` Christophe Leroy [this message]
2021-09-16 14:36 ` David Laight
2021-09-17 12:33 ` Benjamin Herrenschmidt
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=43f736d4-8625-2848-786f-79b902d5c753@csgroup.eu \
--to=christophe.leroy@csgroup.eu \
--cc=benh@kernel.crashing.org \
--cc=fthain@linux-m68k.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linuxppc-dev@lists.ozlabs.org \
--cc=mpe@ellerman.id.au \
--cc=paulus@samba.org \
--cc=userm57@yahoo.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).