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[80.7.220.175]) by smtp.gmail.com with ESMTPSA id z6sm607926wmi.1.2020.10.29.09.39.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 29 Oct 2020 09:39:23 -0700 (PDT) Date: Thu, 29 Oct 2020 16:39:21 +0000 From: Daniel Thompson To: Sumit Garg Cc: maz@kernel.org, catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, tglx@linutronix.de, jason@lakedaemon.net, linux@armlinux.org.uk, tsbogend@alpha.franken.de, mpe@ellerman.id.au, davem@davemloft.net, mingo@redhat.com, bp@alien8.de, x86@kernel.org, mark.rutland@arm.com, julien.thierry.kdev@gmail.com, dianders@chromium.org, jason.wessel@windriver.com, msys.mizuma@gmail.com, ito-yuichi@fujitsu.com, kgdb-bugreport@lists.sourceforge.net, linux-kernel@vger.kernel.org Subject: Re: [PATCH v6 7/7] arm64: kgdb: Roundup cpus using IPI as NMI Message-ID: <20201029163921.dibail374cwwonvo@holly.lan> References: <1603983387-8738-1-git-send-email-sumit.garg@linaro.org> <1603983387-8738-8-git-send-email-sumit.garg@linaro.org> <20201029162234.a5czyjy4eyto6aa4@holly.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201029162234.a5czyjy4eyto6aa4@holly.lan> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > --- > > 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 > > #include > > +#include > > #include > > #include > > > > @@ -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 > > #include > > +#include > > #include > > > > 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.