Re: [PATCH v6 6/7] kgdb: roundup: Allow runtime arch specific override

[Daniel Thompson <daniel.thompson@linaro.org>]

On Thu, Oct 29, 2020 at 08:26:26PM +0530, Sumit Garg wrote:
> Add a new API kgdb_arch_roundup_cpus() for a particular archichecture to
> override default kgdb roundup and if it detects at runtime to not support
> NMI roundup then it can fallback to default implementation using async
> SMP cross-calls.
> 
> Currently such an architecture example is arm64 supporting pseudo NMIs
> feature which is only available on platforms which have support for GICv3
> or later version.
> 
> Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
> ---
>  arch/powerpc/kernel/kgdb.c |  3 ++-
>  arch/sparc/kernel/smp_64.c |  3 ++-
>  arch/x86/kernel/kgdb.c     |  6 ++++--
>  include/linux/kgdb.h       |  5 +++--
>  kernel/debug/debug_core.c  | 10 +++++++++-
>  5 files changed, 20 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/kgdb.c b/arch/powerpc/kernel/kgdb.c
> index 4090802..126575d 100644
> --- a/arch/powerpc/kernel/kgdb.c
> +++ b/arch/powerpc/kernel/kgdb.c
> @@ -125,9 +125,10 @@ static int kgdb_debugger_ipi(struct pt_regs *regs)
>  }
>  
>  #ifdef CONFIG_SMP
> -void kgdb_roundup_cpus(void)
> +bool kgdb_arch_roundup_cpus(void)
>  {
>  	smp_send_debugger_break();
> +	return true;
>  }
>  #endif
>  
> diff --git a/arch/sparc/kernel/smp_64.c b/arch/sparc/kernel/smp_64.c
> index e38d8bf..c459c83 100644
> --- a/arch/sparc/kernel/smp_64.c
> +++ b/arch/sparc/kernel/smp_64.c
> @@ -1014,9 +1014,10 @@ void flush_dcache_page_all(struct mm_struct *mm, struct page *page)
>  }
>  
>  #ifdef CONFIG_KGDB
> -void kgdb_roundup_cpus(void)
> +bool kgdb_arch_roundup_cpus(void)
>  {
>  	smp_cross_call(&xcall_kgdb_capture, 0, 0, 0);
> +	return true;
>  }
>  #endif
>  
> diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
> index ff7878d..1b756d9 100644
> --- a/arch/x86/kernel/kgdb.c
> +++ b/arch/x86/kernel/kgdb.c
> @@ -404,7 +404,8 @@ static void kgdb_disable_hw_debug(struct pt_regs *regs)
>  
>  #ifdef CONFIG_SMP
>  /**
> - *	kgdb_roundup_cpus - Get other CPUs into a holding pattern
> + *	kgdb_arch_roundup_cpus - Get other CPUs into a holding pattern
> + *				 in an architectural specific manner
>   *
>   *	On SMP systems, we need to get the attention of the other CPUs
>   *	and get them be in a known state.  This should do what is needed
> @@ -414,9 +415,10 @@ static void kgdb_disable_hw_debug(struct pt_regs *regs)
>   *
>   *	On non-SMP systems, this is not called.
>   */
> -void kgdb_roundup_cpus(void)
> +bool kgdb_arch_roundup_cpus(void)
>  {
>  	apic_send_IPI_allbutself(NMI_VECTOR);
> +	return true;
>  }
>  #endif
>  
> diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
> index 0d6cf64..f9db5b8 100644
> --- a/include/linux/kgdb.h
> +++ b/include/linux/kgdb.h
> @@ -200,7 +200,8 @@ kgdb_arch_handle_qxfer_pkt(char *remcom_in_buffer,
>  extern void kgdb_call_nmi_hook(void *ignored);
>  
>  /**
> - *	kgdb_roundup_cpus - Get other CPUs into a holding pattern
> + *	kgdb_arch_roundup_cpus - Get other CPUs into a holding pattern
> + *				 in an architectural specific manner
>   *
>   *	On SMP systems, we need to get the attention of the other CPUs
>   *	and get them into a known state.  This should do what is needed
> @@ -210,7 +211,7 @@ extern void kgdb_call_nmi_hook(void *ignored);
>   *
>   *	On non-SMP systems, this is not called.
>   */
> -extern void kgdb_roundup_cpus(void);
> +extern bool kgdb_arch_roundup_cpus(void);
>  
>  /**
>   *	kgdb_arch_set_pc - Generic call back to the program counter
> diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
> index 1e75a89..27e401c 100644
> --- a/kernel/debug/debug_core.c
> +++ b/kernel/debug/debug_core.c
> @@ -241,13 +241,21 @@ void __weak kgdb_call_nmi_hook(void *ignored)
>  }
>  NOKPROBE_SYMBOL(kgdb_call_nmi_hook);
>  
> -void __weak kgdb_roundup_cpus(void)
> +bool __weak kgdb_arch_roundup_cpus(void)
> +{
> +	return false;

Do we really need to introduce all these boolean return values just to
call a bit of library code when one of the architectures wants to
fallback to a generic implementation?

Why not something more like:

void kgdb_smp_call_nmi_hook(void)
{
    /* original weak version of kgdb_roundup_cpus goes here */
}

void __weak kgdb_roundup_cpus(void)
{
    kgdb_smp_call_nmi_hook();
}

arm64 can now simply call the new library function if a fallback is needed. 

Note that same question applies to the backtrace changes...


Daniel.


> +}
> +
> +static void kgdb_roundup_cpus(void)
>  {
>  	call_single_data_t *csd;
>  	int this_cpu = raw_smp_processor_id();
>  	int cpu;
>  	int ret;
>  
> +	if (kgdb_arch_roundup_cpus())
> +		return;
> +
>  	for_each_online_cpu(cpu) {
>  		/* No need to roundup ourselves */
>  		if (cpu == this_cpu)
> -- 
> 2.7.4
>