[PATCH] powerpc/asm: Mark cr0 as clobbered in mftb()

[PATCH] powerpc/asm: Mark cr0 as clobbered in mftb()

[Oliver O'Halloran]

The workaround for the CELL timebase bug does not correctly mark cr0 as
being clobbered. This can result in GCC making some poor^W completely
broken optimisations.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
---
 arch/powerpc/include/asm/reg.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h
index 7e50e47375d6..a3b6575c7842 100644
--- a/arch/powerpc/include/asm/reg.h
+++ b/arch/powerpc/include/asm/reg.h
@@ -1303,7 +1303,7 @@ static inline void msr_check_and_clear(unsigned long bits)
 				"	.llong 0\n"			\
 				".previous"				\
 			: "=r" (rval) \
-			: "i" (CPU_FTR_CELL_TB_BUG), "i" (SPRN_TBRL)); \
+			: "i" (CPU_FTR_CELL_TB_BUG), "i" (SPRN_TBRL) : "cr0"); \
 			rval;})
 #else
 #define mftb()		({unsigned long rval;	\
-- 
2.9.4

Re: [PATCH] powerpc/asm: Mark cr0 as clobbered in mftb()

[Michael Ellerman]

Oliver O'Halloran <oohall@gmail.com> writes:

> The workaround for the CELL timebase bug does not correctly mark cr0 as
> being clobbered. This can result in GCC making some poor^W completely
> broken optimisations.

Fruit 'n oats, how did we never notice that? Luck I guess. Or subtle
breakage that no one could pin down :E

I'll tag it for stable.

Your change log is not entirely fair, it's not GCC's fault, we changed
register state without telling it, so it's on us :)  I'll reword it a
bit here.

cheers

> diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h
> index 7e50e47375d6..a3b6575c7842 100644
> --- a/arch/powerpc/include/asm/reg.h
> +++ b/arch/powerpc/include/asm/reg.h
> @@ -1303,7 +1303,7 @@ static inline void msr_check_and_clear(unsigned long bits)
>  				"	.llong 0\n"			\
>  				".previous"				\
>  			: "=r" (rval) \
> -			: "i" (CPU_FTR_CELL_TB_BUG), "i" (SPRN_TBRL)); \
> +			: "i" (CPU_FTR_CELL_TB_BUG), "i" (SPRN_TBRL) : "cr0"); \
>  			rval;})
>  #else
>  #define mftb()		({unsigned long rval;	\
> -- 
> 2.9.4

Re: [PATCH] powerpc/asm: Mark cr0 as clobbered in mftb()

[Segher Boessenkool]

Hi!

On Mon, Jul 10, 2017 at 09:50:06PM +1000, Michael Ellerman wrote:
> Oliver O'Halloran <oohall@gmail.com> writes:
> 
> > The workaround for the CELL timebase bug does not correctly mark cr0 as
> > being clobbered. This can result in GCC making some poor^W completely
> > broken optimisations.
> 
> Fruit 'n oats, how did we never notice that? Luck I guess. Or subtle
> breakage that no one could pin down :E

GCC does not use cr0 before it has used cr7, cr5, cr6, cr1 (unless some
instruction _requires_ cr0, like record form ("dot") instructions, which
until recently were disabled when targetting Cell), so it isn't too easy
to hit the problem here.  Maybe Oliver used a very new GCC?  Or he was
unlucky.

> I'll tag it for stable.
> 
> Your change log is not entirely fair, it's not GCC's fault, we changed
> register state without telling it, so it's on us :)  I'll reword it a
> bit here.

Yep, GCC works fine here, GIGO etc.  It isn't feasible to make GCC warn
for most inline asm problems, either (we do not parse the mnemonics in
the asm; this is a fundamental part of the design; you must express all
constraints as, well, constraints).


Segher

Re: [PATCH] powerpc/asm: Mark cr0 as clobbered in mftb()

[Michael Ellerman]

Segher Boessenkool <segher@kernel.crashing.org> writes:

> Hi!
>
> On Mon, Jul 10, 2017 at 09:50:06PM +1000, Michael Ellerman wrote:
>> Oliver O'Halloran <oohall@gmail.com> writes:
>> 
>> > The workaround for the CELL timebase bug does not correctly mark cr0 as
>> > being clobbered. This can result in GCC making some poor^W completely
>> > broken optimisations.
>> 
>> Fruit 'n oats, how did we never notice that? Luck I guess. Or subtle
>> breakage that no one could pin down :E
>
> GCC does not use cr0 before it has used cr7, cr5, cr6, cr1 (unless some
> instruction _requires_ cr0, like record form ("dot") instructions, which
> until recently were disabled when targetting Cell), so it isn't too easy
> to hit the problem here.  Maybe Oliver used a very new GCC?  Or he was
> unlucky.

Aha. I think he actually hit the bug elsewhere and then noticed it here
also? And yeah I think it was a dot instruction that triggered it.

>> I'll tag it for stable.
>> 
>> Your change log is not entirely fair, it's not GCC's fault, we changed
>> register state without telling it, so it's on us :)  I'll reword it a
>> bit here.
>
> Yep, GCC works fine here, GIGO etc.  It isn't feasible to make GCC warn
> for most inline asm problems, either (we do not parse the mnemonics in
> the asm; this is a fundamental part of the design; you must express all
> constraints as, well, constraints).

Ack. It's a terrible terrible interface, but that's the price we pay for
wanting to insert arbitrary bits of asm inside an otherwise compiled
language :)

When I get the time I might go through our inline asm and convert all
the cmpwi x,z to cmpwi cr0,x,z, so at least the cr0 usage is a little
more obvious.

cheers

Re: [PATCH] powerpc/asm: Mark cr0 as clobbered in mftb()

[Oliver]

On Mon, Jul 10, 2017 at 9:50 PM, Michael Ellerman <mpe@ellerman.id.au> wrote:
> Oliver O'Halloran <oohall@gmail.com> writes:
>
>> The workaround for the CELL timebase bug does not correctly mark cr0 as
>> being clobbered. This can result in GCC making some poor^W completely
>> broken optimisations.
>
> Fruit 'n oats, how did we never notice that? Luck I guess. Or subtle
> breakage that no one could pin down :E

Dumb luck probably.  The workaround is inside a feature section which
depends on CPU_FTR_CELL_TB_BUG so you would only ever see a problem
when actually running on Cell (can't be that many people...). CR0
being volatile across function calls probably helped mask it too.

> I'll tag it for stable.
Good idea.

> Your change log is not entirely fair, it's not GCC's fault, we changed
> register state without telling it, so it's on us :)  I'll reword it a
> bit here.

Yeah that was probably a bit harsh.

>
> cheers
>
>> diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h
>> index 7e50e47375d6..a3b6575c7842 100644
>> --- a/arch/powerpc/include/asm/reg.h
>> +++ b/arch/powerpc/include/asm/reg.h
>> @@ -1303,7 +1303,7 @@ static inline void msr_check_and_clear(unsigned long bits)
>>                               "       .llong 0\n"                     \
>>                               ".previous"                             \
>>                       : "=r" (rval) \
>> -                     : "i" (CPU_FTR_CELL_TB_BUG), "i" (SPRN_TBRL)); \
>> +                     : "i" (CPU_FTR_CELL_TB_BUG), "i" (SPRN_TBRL) : "cr0"); \
>>                       rval;})
>>  #else
>>  #define mftb()               ({unsigned long rval;   \
>> --
>> 2.9.4

Re: powerpc/asm: Mark cr0 as clobbered in mftb()

[Michael Ellerman]

On Thu, 2017-07-06 at 08:46:43 UTC, Oliver O'Halloran wrote:
> The workaround for the CELL timebase bug does not correctly mark cr0 as
> being clobbered. This can result in GCC making some poor^W completely
> broken optimisations.
> 
> Signed-off-by: Oliver O'Halloran <oohall@gmail.com>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/2400fd822f467cb4c886c879d8ad99

cheers