Re: [PATCH] powerpc: warn on emulation of dcbz instruction
On Thu, 2021-09-16 at 14:36 +, David Laight wrote: > > Does userspace accesses non-cached memory directly ? > > > It probably can if a driver mmaps PCI space directly into user space. > > That certainly works on x86-64. The posterchild for that is Xorg Cheers, Ben.
RE: [PATCH] powerpc: warn on emulation of dcbz instruction
From: Christophe Leroy > Sent: 16 September 2021 08:24 > > 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 > >>> Cc: Finn Thain > >>> Signed-off-by: Christophe Leroy > >>> --- > >>> 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 ? It probably can if a driver mmaps PCI space directly into user space. That certainly works on x86-64. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
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 Cc: Finn Thain Signed-off-by: Christophe Leroy --- 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
Re: [PATCH] powerpc: warn on emulation of dcbz instruction
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 > > Cc: Finn Thain > > Signed-off-by: Christophe Leroy > > --- > > 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 ? Ah I saw your other one about fbdev... Ok what about you do that in a if (!user_mode(regs)) ? Indeed the kernel should not do that. Cheers, Ben.
Re: [PATCH] powerpc: warn on emulation of dcbz instruction
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 > Cc: Finn Thain > Signed-off-by: Christophe Leroy > --- > 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 ? Ben.
