Re: [PATCH] [v2] coresight: etm4x: avoid build failure with unrolled loops

[Suzuki K Poulose]

On 29/04/2021 18:50, Arnd Bergmann wrote:
> On Thu, Apr 29, 2021 at 7:37 PM Suzuki K Poulose <suzuki.poulose@xxxxxxx> wrote:
> > On 29/04/2021 15:57, Arnd Bergmann wrote:
> > > From: Arnd Bergmann <arnd@xxxxxxxx>
> > >
> > > clang-12 fails to build the etm4x driver with -fsanitize=array-bounds,
> > > where it decides to unroll certain loops in a way that result in a
> > > C variable getting put into an inline assembly
> > >
> > > <instantiation>:1:7: error: expected constant expression in '.inst' directive
> > > .inst (0xd5200000|((((2) << 19) | ((1) << 16) | (((((((((((0x160 + (i * 4))))) >> 2))) >> 7) & 0x7)) << 12) | ((((((((((0x160 + (i * 4))))) >> 2))) & 0xf)) << 8) | (((((((((((0x160 + (i * 4))))) >> 2))) >> 4) & 0x7)) << 5)))|(.L__reg_num_x8))
> > >         ^
> > > drivers/hwtracing/coresight/coresight-etm4x-core.c:702:4: note: while in macro instantiation
> > >                           etm4x_relaxed_read32(csa, TRCCNTVRn(i));
> > >                           ^
> > > drivers/hwtracing/coresight/coresight-etm4x.h:403:4: note: expanded from macro 'etm4x_relaxed_read32'
> > >                    read_etm4x_sysreg_offset((offset), false)))
> > >                    ^
> > > drivers/hwtracing/coresight/coresight-etm4x.h:383:12: note: expanded from macro 'read_etm4x_sysreg_offset'
> > >                           __val = read_etm4x_sysreg_const_offset((offset));       \
> > >                                   ^
> > > drivers/hwtracing/coresight/coresight-etm4x.h:149:2: note: expanded from macro 'read_etm4x_sysreg_const_offset'
> > >           READ_ETM4x_REG(ETM4x_OFFSET_TO_REG(offset))
> > >           ^
> > > drivers/hwtracing/coresight/coresight-etm4x.h:144:2: note: expanded from macro 'READ_ETM4x_REG'
> > >           read_sysreg_s(ETM4x_REG_NUM_TO_SYSREG((reg)))
> > >           ^
> > > arch/arm64/include/asm/sysreg.h:1108:15: note: expanded from macro 'read_sysreg_s'
> > >           asm volatile(__mrs_s("%0", r) : "=r" (__val));                  \
> > >                        ^
> > > arch/arm64/include/asm/sysreg.h:1074:2: note: expanded from macro '__mrs_s'
> > > "       mrs_s " v ", " __stringify(r) "\n"                      \
> > >    ^
> > >
> > > This only happened in a few loops in which the array bounds sanitizer
> > > added a special case for an array overflow that clang determined to be
> > > possible, but any compiler is free to unroll any of the loops in the
> > > same way that breaks the sysreg macros.
> > >
> > > Introduce helper functions that perform a sysreg access with a
> > > non-constant register number and use them in each call that passes
> > > a loop counter.
> >
> > You don't need to add this special helper. We have the exact
> > infrastructure already. So these could simply be replaced with:
> >
> > csdev_access_xxx(csa, ...)
> >
> > see :
> >
> > include/linux/coresight.h
>
> Ah, nice!
>
> Do you mean replacing only the ones that use a nonconstant
> offset, or all of them? I guess changing all would avoid some

Only the CLANG "nonconstant" please. Going through an indirect
function call and large "switch..case" for every single register
access is painful. Which is why I decided to add those ugly macros.

So, please leave the rest as they are.

Suzuki


> really ugly magic macros, but the indirect function call and the
> switch() adds a few cycles of overhead every time and the code
> looks like it is micro-optimized for fast register access here.
>
>        Arnd