Messages in this thread | | | Subject | Re: [PATCH] powerpc: warn on emulation of dcbz instruction | From | Christophe Leroy <> | Date | Thu, 16 Sep 2021 09:23:57 +0200 |
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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
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