Re: [PATCH v6 7/7] arm64: kgdb: Roundup cpus using IPI as NMI

[Daniel Thompson]

On Thu, Oct 29, 2020 at 04:22:34PM +0000, Daniel Thompson wrote:
> On Thu, Oct 29, 2020 at 08:26:27PM +0530, Sumit Garg wrote:
> > arm64 platforms with GICv3 or later supports pseudo NMIs which can be
> > leveraged to roundup CPUs which are stuck in hard lockup state with
> > interrupts disabled that wouldn't be possible with a normal IPI.
> > 
> > So instead switch to roundup CPUs using IPI turned as NMI. And in
> > case a particular arm64 platform doesn't supports pseudo NMIs,
> > it will switch back to default kgdb CPUs roundup mechanism.
> > 
> > Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/kgdb.h |  9 +++++++++
> >  arch/arm64/kernel/ipi_nmi.c   |  5 +++++
> >  arch/arm64/kernel/kgdb.c      | 35 +++++++++++++++++++++++++++++++++++
> >  3 files changed, 49 insertions(+)
> > 
> > diff --git a/arch/arm64/include/asm/kgdb.h b/arch/arm64/include/asm/kgdb.h
> > index 21fc85e..c3d2425 100644
> > --- a/arch/arm64/include/asm/kgdb.h
> > +++ b/arch/arm64/include/asm/kgdb.h
> > @@ -24,6 +24,15 @@ static inline void arch_kgdb_breakpoint(void)
> >  extern void kgdb_handle_bus_error(void);
> >  extern int kgdb_fault_expected;
> >  
> > +#ifdef CONFIG_KGDB
> > +extern bool kgdb_ipi_nmicallback(int cpu, void *regs);
> > +#else
> > +static inline bool kgdb_ipi_nmicallback(int cpu, void *regs)
> > +{
> > +	return false;
> > +}
> > +#endif
> > +
> >  #endif /* !__ASSEMBLY__ */
> >  
> >  /*
> > diff --git a/arch/arm64/kernel/ipi_nmi.c b/arch/arm64/kernel/ipi_nmi.c
> > index 597dcf7..6ace182 100644
> > --- a/arch/arm64/kernel/ipi_nmi.c
> > +++ b/arch/arm64/kernel/ipi_nmi.c
> > @@ -8,6 +8,7 @@
> >  
> >  #include <linux/interrupt.h>
> >  #include <linux/irq.h>
> > +#include <linux/kgdb.h>
> >  #include <linux/nmi.h>
> >  #include <linux/smp.h>
> >  
> > @@ -45,10 +46,14 @@ bool arch_trigger_cpumask_backtrace(const cpumask_t *mask, bool exclude_self)
> >  static irqreturn_t ipi_nmi_handler(int irq, void *data)
> >  {
> >  	irqreturn_t ret = IRQ_NONE;
> > +	unsigned int cpu = smp_processor_id();
> >  
> >  	if (nmi_cpu_backtrace(get_irq_regs()))
> >  		ret = IRQ_HANDLED;
> >  
> > +	if (kgdb_ipi_nmicallback(cpu, get_irq_regs()))
> > +		ret = IRQ_HANDLED;
> > +
> >  	return ret;
> 
> It would be better to declare existing return value for
> kgdb_nmicallback() to be dangerously stupid and fix it so it returns an
> irqreturn_t (that's easy since most callers do not need to check the
> return value).
> 
> Then this code simply becomes:
> 
> 	return kgdb_nmicallback(cpu, get_irq_regs());

Actually, reflecting on this maybe it is better to keep kgdb_nmicallin()
and kgdb_nmicallback() aligned w.r.t. return codes (even if they are a
little unusual).

I'm still not sure why we'd keep kgdb_ipi_nmicallback() though.
kgdb_nmicallback() is intended to be called from arch code...


Daniel.


> 
> 
> >  }
> >  
> > diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
> > index 1a157ca3..c26e710 100644
> > --- a/arch/arm64/kernel/kgdb.c
> > +++ b/arch/arm64/kernel/kgdb.c
> > @@ -17,6 +17,7 @@
> >  
> >  #include <asm/debug-monitors.h>
> >  #include <asm/insn.h>
> > +#include <asm/nmi.h>
> >  #include <asm/traps.h>
> >  
> >  struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = {
> > @@ -353,3 +354,37 @@ int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
> >  	return aarch64_insn_write((void *)bpt->bpt_addr,
> >  			*(u32 *)bpt->saved_instr);
> >  }
> > +
> > +bool kgdb_ipi_nmicallback(int cpu, void *regs)
> > +{
> > +	if (atomic_read(&kgdb_active) != -1) {
> > +		kgdb_nmicallback(cpu, regs);
> > +		return true;
> > +	}
> > +
> > +	return false;
> > +}
> 
> I *really* don't like this function.
> 
> If the return code of kgdb_nmicallback() is broken then fix it, don't
> just wrap it and invent a new criteria for the return code.
> 
> To be honest I don't actually think the logic in kgdb_nmicallback() is
> broken. As mentioned above the return value has a weird definition (0
> for "handled it OK" and 1 for "nothing for me to do") but the logic to
> calculate the return code looks OK.
> 
> 
> > +
> > +static void kgdb_smp_callback(void *data)
> > +{
> > +	unsigned int cpu = smp_processor_id();
> > +
> > +	if (atomic_read(&kgdb_active) != -1)
> > +		kgdb_nmicallback(cpu, get_irq_regs());
> > +}
> 
> This is Unused. I presume it is litter from a previous revision of the
> code and can be deleted?
> 
> 
> > +
> > +bool kgdb_arch_roundup_cpus(void)
> > +{
> > +	struct cpumask mask;
> > +
> > +	if (!arm64_supports_nmi())
> > +		return false;
> > +
> > +	cpumask_copy(&mask, cpu_online_mask);
> > +	cpumask_clear_cpu(raw_smp_processor_id(), &mask);
> > +	if (cpumask_empty(&mask))
> > +		return false;
> 
> Why do we need to fallback if there is no work to do? There will still
> be no work to do when we call the fallback.
> 
> 
> Daniel.