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

[Christophe Leroy]

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@xxxxxxxxx>
> > > Cc: Finn Thain <fthain@xxxxxxxxxxxxxx>
> > > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > > ---
> > >   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