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

[Sumit Garg]

On Thu, 29 Oct 2020 at 20:51, Daniel Thompson
<daniel.thompson@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
> > ---
> >  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.
>

Makes sense, I will use this approach instead.

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

In case of backtrace, there are already multiple APIs wrapping
arch_trigger_cpumask_backtrace() as follows:

- trigger_all_cpu_backtrace()
- trigger_allbutself_cpu_backtrace()
- trigger_cpumask_backtrace()
- trigger_single_cpu_backtrace()

And each of them already return a boolean and have corresponding
different fallback mechanisms. So we can't have a common fallback API
from arch specific code and instead need to extend that boolean return
to arch specific code that is implemented as part of patch #4.

If you do have any better ideas then do let me know.

-Sumit

>
>
> 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
> >