Re: [PATCH 2/2] riscv: Disable misaligned access probe when CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS

[Charlie Jenkins <charlie@rivosinc.com>]

On Thu, Feb 01, 2024 at 02:43:43PM +0100, Clément Léger wrote:
> 
> 
> On 01/02/2024 07:40, Charlie Jenkins wrote:
> > When CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is selected, the cpus can be
> > set to have fast misaligned access without needing to probe.
> > 
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > ---
> >  arch/riscv/include/asm/cpufeature.h  | 7 +++++++
> >  arch/riscv/kernel/cpufeature.c       | 4 ++++
> >  arch/riscv/kernel/sys_hwprobe.c      | 4 ++++
> >  arch/riscv/kernel/traps_misaligned.c | 4 ++++
> >  4 files changed, 19 insertions(+)
> > 
> > diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
> > index dfdcca229174..7d8d64783e38 100644
> > --- a/arch/riscv/include/asm/cpufeature.h
> > +++ b/arch/riscv/include/asm/cpufeature.h
> > @@ -137,10 +137,17 @@ static __always_inline bool riscv_cpu_has_extension_unlikely(int cpu, const unsi
> >  	return __riscv_isa_extension_available(hart_isa[cpu].isa, ext);
> >  }
> >  
> > +#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> >  DECLARE_STATIC_KEY_FALSE(fast_misaligned_access_speed_key);
> >  
> >  static __always_inline bool has_fast_misaligned_accesses(void)
> >  {
> >  	return static_branch_likely(&fast_misaligned_access_speed_key);
> >  }
> > +#else
> > +static __always_inline bool has_fast_misaligned_accesses(void)
> > +{
> > +	return true;
> > +}
> > +#endif
> >  #endif
> > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > index 89920f84d0a3..d787846c0b68 100644
> > --- a/arch/riscv/kernel/cpufeature.c
> > +++ b/arch/riscv/kernel/cpufeature.c
> > @@ -43,10 +43,12 @@ static DECLARE_BITMAP(riscv_isa, RISCV_ISA_EXT_MAX) __read_mostly;
> >  /* Per-cpu ISA extensions. */
> >  struct riscv_isainfo hart_isa[NR_CPUS];
> >  
> > +#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> >  /* Performance information */
> >  DEFINE_PER_CPU(long, misaligned_access_speed);
> >  
> >  static cpumask_t fast_misaligned_access;
> > +#endif
> >  
> >  /**
> >   * riscv_isa_extension_base() - Get base extension word
> > @@ -706,6 +708,7 @@ unsigned long riscv_get_elf_hwcap(void)
> >  	return hwcap;
> >  }
> >  
> > +#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> >  static int check_unaligned_access(void *param)
> >  {
> >  	int cpu = smp_processor_id();
> > @@ -946,6 +949,7 @@ static int check_unaligned_access_all_cpus(void)
> >  }
> >  
> >  arch_initcall(check_unaligned_access_all_cpus);
> > +#endif /* CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS */
> >  
> >  void riscv_user_isa_enable(void)
> >  {
> 
> Hi Charlie,
> 
> Generally, having so much ifdef in various pieces of code is probably
> not a good idea.
> 
> AFAICT, if CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is enabled, the whole
> misaligned access speed checking could be opt-out. which means that
> probably everything related to misaligned accesses should be moved in
> it's own file build it only for CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=n
> only.

I will look into doing something more clever here! I agree it is not
very nice to have so many ifdefs scattered.

> 
> > diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c
> > index a7c56b41efd2..3f1a6edfdb08 100644
> > --- a/arch/riscv/kernel/sys_hwprobe.c
> > +++ b/arch/riscv/kernel/sys_hwprobe.c
> > @@ -149,6 +149,7 @@ static bool hwprobe_ext0_has(const struct cpumask *cpus, unsigned long ext)
> >  
> >  static u64 hwprobe_misaligned(const struct cpumask *cpus)
> >  {
> > +#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> >  	int cpu;
> >  	u64 perf = -1ULL;
> >  
> > @@ -168,6 +169,9 @@ static u64 hwprobe_misaligned(const struct cpumask *cpus)
> >  		return RISCV_HWPROBE_MISALIGNED_UNKNOWN;
> >  
> >  	return perf;
> > +#else
> > +	return RISCV_HWPROBE_MISALIGNED_FAST;
> > +#endif
> >  }
> >  
> >  static void hwprobe_one_pair(struct riscv_hwprobe *pair,
> > diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned> index 8ded225e8c5b..c24f79d769f6 100644
> > --- a/arch/riscv/kernel/traps_misaligned.c
> > +++ b/arch/riscv/kernel/traps_misaligned.c
> > @@ -413,7 +413,9 @@ int handle_misaligned_load(struct pt_regs *regs)
> >  
> >  	perf_sw_event(PERF_COUNT_SW_ALIGNMENT_FAULTS, 1, regs, addr);
> >  
> > +#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> >  	*this_cpu_ptr(&misaligned_access_speed) = RISCV_HWPROBE_MISALIGNED_EMULATED;
> > +#endif
> 
> I think that rather using ifdefery inside this file (traps_misaligned.c)
>  it can be totally opt-out in case we have
> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS since it implies that misaligned
> accesses are not emulated (at least that is my understanding).
> 

That's a great idea, I believe that is correct.

- Charlie

> Thanks,
> 
> Clément
> 
> 
> >  
> >  	if (!unaligned_enabled)
> >  		return -1;
> > @@ -596,6 +598,7 @@ int handle_misaligned_store(struct pt_regs *regs)
> >  	return 0;
> >  }
> >  
> > +#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> >  bool check_unaligned_access_emulated(int cpu)
> >  {
> >  	long *mas_ptr = per_cpu_ptr(&misaligned_access_speed, cpu);
> > @@ -640,6 +643,7 @@ void unaligned_emulation_finish(void)
> >  	}
> >  	unaligned_ctl = true;
> >  }
> > +#endif
> >  
> >  bool unaligned_ctl_available(void)
> >  {
> > 
> 
> 
>