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.g...@linaro.org>
> ---
> 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());
> }
>
> 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.
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