Re: [PATCH v6 15/24] arm64: Switch to PMR masking when starting CPUs
On 04/12/18 17:51, Catalin Marinas wrote:
> On Mon, Nov 12, 2018 at 11:57:06AM +, Julien Thierry wrote:
>> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
>> index 8dc9dde..e495360 100644
>> --- a/arch/arm64/kernel/smp.c
>> +++ b/arch/arm64/kernel/smp.c
>> @@ -35,6 +35,7 @@
>> #include
>> #include
>> #include
>> +#include
>> #include
>> #include
>> #include
>> @@ -175,6 +176,25 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
>> return ret;
>> }
>>
>> +static void init_gic_priority_masking(void)
>> +{
>> +u32 gic_sre = gic_read_sre();
>> +u32 cpuflags;
>> +
>> +if (WARN_ON(!(gic_sre & ICC_SRE_EL1_SRE)))
>> +return;
>> +
>> +WARN_ON(!irqs_disabled());
>> +
>> +gic_write_pmr(GIC_PRIO_IRQOFF);
>> +
>> +cpuflags = read_sysreg(daif);
>> +
>> +/* We can only unmask PSR.I if we can take aborts */
>> +if (!(cpuflags & PSR_A_BIT))
>> +write_sysreg(cpuflags & ~PSR_I_BIT, daif);
>
> I don't understand this. If you don't switch off PSR_I_BIT here, where
> does it happen? In which scenario do we actually have the A bit still
> set? At a quick look, smp_prepare_boot_cpu() would have the A bit
> cleared previously by setup_arch(). We have secondary_start_kernel()
> where you call init_gic_priority_masking() before local_daif_restore().
>
So this is for secondary CPUs where PSR.A can be still set.
The thing is that the daifflags.h establishes the order for disabling
types of exceptions:
Debug > Abort > IRQ
The idea is that when introducing pseudo-NMIs this becomes:
Debug > Abort > pseudo-NMI > IRQ
Whenever aborts are disabled (maybe because we just took an abort) we
don't want to take an NMI.
> So what happens if you always turn off PSR_I_BIT here?
>
So semantically it would be saying "we can take a pseudo-NMI here".
Realistically, I think it depends on the state of the GIC redistributor
for this CPU:
- If the re-distributor was initialized, nothing bad could happen as no
NMI could have been configured for this CPU yet.
- If the re-distributor initialization is done between the call to
init_gic_priority_mask() and the local_daif_restore() then probably bad
things could happen
I can try to figure out if it is safe to just clear PSR.I always, but I
also find it easier to always play by the rule "if PSR.A is set, PSR.I
is set".
Thanks,
--
Julien Thierry
Re: [PATCH v6 15/24] arm64: Switch to PMR masking when starting CPUs
On 04/12/18 17:51, Catalin Marinas wrote:
> On Mon, Nov 12, 2018 at 11:57:06AM +, Julien Thierry wrote:
>> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
>> index 8dc9dde..e495360 100644
>> --- a/arch/arm64/kernel/smp.c
>> +++ b/arch/arm64/kernel/smp.c
>> @@ -35,6 +35,7 @@
>> #include
>> #include
>> #include
>> +#include
>> #include
>> #include
>> #include
>> @@ -175,6 +176,25 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
>> return ret;
>> }
>>
>> +static void init_gic_priority_masking(void)
>> +{
>> +u32 gic_sre = gic_read_sre();
>> +u32 cpuflags;
>> +
>> +if (WARN_ON(!(gic_sre & ICC_SRE_EL1_SRE)))
>> +return;
>> +
>> +WARN_ON(!irqs_disabled());
>> +
>> +gic_write_pmr(GIC_PRIO_IRQOFF);
>> +
>> +cpuflags = read_sysreg(daif);
>> +
>> +/* We can only unmask PSR.I if we can take aborts */
>> +if (!(cpuflags & PSR_A_BIT))
>> +write_sysreg(cpuflags & ~PSR_I_BIT, daif);
>
> I don't understand this. If you don't switch off PSR_I_BIT here, where
> does it happen? In which scenario do we actually have the A bit still
> set? At a quick look, smp_prepare_boot_cpu() would have the A bit
> cleared previously by setup_arch(). We have secondary_start_kernel()
> where you call init_gic_priority_masking() before local_daif_restore().
>
So this is for secondary CPUs where PSR.A can be still set.
The thing is that the daifflags.h establishes the order for disabling
types of exceptions:
Debug > Abort > IRQ
The idea is that when introducing pseudo-NMIs this becomes:
Debug > Abort > pseudo-NMI > IRQ
Whenever aborts are disabled (maybe because we just took an abort) we
don't want to take an NMI.
> So what happens if you always turn off PSR_I_BIT here?
>
So semantically it would be saying "we can take a pseudo-NMI here".
Realistically, I think it depends on the state of the GIC redistributor
for this CPU:
- If the re-distributor was initialized, nothing bad could happen as no
NMI could have been configured for this CPU yet.
- If the re-distributor initialization is done between the call to
init_gic_priority_mask() and the local_daif_restore() then probably bad
things could happen
I can try to figure out if it is safe to just clear PSR.I always, but I
also find it easier to always play by the rule "if PSR.A is set, PSR.I
is set".
Thanks,
--
Julien Thierry
Re: [PATCH v6 15/24] arm64: Switch to PMR masking when starting CPUs
On Mon, Nov 12, 2018 at 11:57:06AM +, Julien Thierry wrote:
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 8dc9dde..e495360 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -35,6 +35,7 @@
> #include
> #include
> #include
> +#include
> #include
> #include
> #include
> @@ -175,6 +176,25 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
> return ret;
> }
>
> +static void init_gic_priority_masking(void)
> +{
> + u32 gic_sre = gic_read_sre();
> + u32 cpuflags;
> +
> + if (WARN_ON(!(gic_sre & ICC_SRE_EL1_SRE)))
> + return;
> +
> + WARN_ON(!irqs_disabled());
> +
> + gic_write_pmr(GIC_PRIO_IRQOFF);
> +
> + cpuflags = read_sysreg(daif);
> +
> + /* We can only unmask PSR.I if we can take aborts */
> + if (!(cpuflags & PSR_A_BIT))
> + write_sysreg(cpuflags & ~PSR_I_BIT, daif);
I don't understand this. If you don't switch off PSR_I_BIT here, where
does it happen? In which scenario do we actually have the A bit still
set? At a quick look, smp_prepare_boot_cpu() would have the A bit
cleared previously by setup_arch(). We have secondary_start_kernel()
where you call init_gic_priority_masking() before local_daif_restore().
So what happens if you always turn off PSR_I_BIT here?
--
Catalin
Re: [PATCH v6 15/24] arm64: Switch to PMR masking when starting CPUs
On Mon, Nov 12, 2018 at 11:57:06AM +, Julien Thierry wrote:
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 8dc9dde..e495360 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -35,6 +35,7 @@
> #include
> #include
> #include
> +#include
> #include
> #include
> #include
> @@ -175,6 +176,25 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
> return ret;
> }
>
> +static void init_gic_priority_masking(void)
> +{
> + u32 gic_sre = gic_read_sre();
> + u32 cpuflags;
> +
> + if (WARN_ON(!(gic_sre & ICC_SRE_EL1_SRE)))
> + return;
> +
> + WARN_ON(!irqs_disabled());
> +
> + gic_write_pmr(GIC_PRIO_IRQOFF);
> +
> + cpuflags = read_sysreg(daif);
> +
> + /* We can only unmask PSR.I if we can take aborts */
> + if (!(cpuflags & PSR_A_BIT))
> + write_sysreg(cpuflags & ~PSR_I_BIT, daif);
I don't understand this. If you don't switch off PSR_I_BIT here, where
does it happen? In which scenario do we actually have the A bit still
set? At a quick look, smp_prepare_boot_cpu() would have the A bit
cleared previously by setup_arch(). We have secondary_start_kernel()
where you call init_gic_priority_masking() before local_daif_restore().
So what happens if you always turn off PSR_I_BIT here?
--
Catalin
