Re: [PATCH v2 1/6] arm64: cpufeature: Allow early detect of specific features

[Julien Thierry]

On 22/01/18 12:05, Suzuki K Poulose wrote:
> On 17/01/18 11:54, Julien Thierry wrote:
> > From: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> >
> > Currently it is not possible to detect features of the boot CPU
> > until the other CPUs have been brought up.
> >
> > This prevents us from reacting to features of the boot CPU until
> > fairly late in the boot process. To solve this we allow a subset
> > of features (that are likely to be common to all clusters) to be
> > detected based on the boot CPU alone.
> >
> > Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> > [julien.thierry@xxxxxxx: check non-boot cpu missing early features, avoid
> >              duplicates between early features and normal
> >              features]
> > Signed-off-by: Julien Thierry <julien.thierry@xxxxxxx>
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> > ---
> >   arch/arm64/kernel/cpufeature.c | 69 
> > ++++++++++++++++++++++++++++--------------
> >   1 file changed, 47 insertions(+), 22 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/cpufeature.c 
> > b/arch/arm64/kernel/cpufeature.c
> > index a73a592..6698404 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -52,6 +52,8 @@
> >   DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
> >   EXPORT_SYMBOL(cpu_hwcaps);
> >
> > +static void __init setup_early_feature_capabilities(void);
> > +
> >   /*
> >    * Flag to indicate if we have computed the system wide
> >    * capabilities based on the boot time active CPUs. This
> > @@ -542,6 +544,8 @@ void __init init_cpu_features(struct cpuinfo_arm64 
> > *info)
> >           init_cpu_ftr_reg(SYS_ZCR_EL1, info->reg_zcr);
> >           sve_init_vq_map();
> >       }
> > +
> > +    setup_early_feature_capabilities();
> >   }
> >
> >   static void update_cpu_ftr_reg(struct arm64_ftr_reg *reg, u64 new)
> > @@ -846,7 +850,7 @@ static bool has_no_fpsimd(const struct 
> > arm64_cpu_capabilities *entry, int __unus
> >                       ID_AA64PFR0_FP_SHIFT) < 0;
> >   }
> >
> > -static const struct arm64_cpu_capabilities arm64_features[] = {
> > +static const struct arm64_cpu_capabilities arm64_early_features[] = {
> >       {
> >           .desc = "GIC system register CPU interface",
> >           .capability = ARM64_HAS_SYSREG_GIC_CPUIF,
> > @@ -857,6 +861,10 @@ static bool has_no_fpsimd(const struct 
> > arm64_cpu_capabilities *entry, int __unus
> >           .sign = FTR_UNSIGNED,
> >           .min_field_value = 1,
> >       },
> > +    {}
> > +};
> > +
>
>
> Julien,
>
> One potential problem with this is that we don't have a way
> to make this work on a "theoretical" system with and without
> GIC system reg interface. i.e, if we don't have the CONFIG
> enabled for using ICC system regs for IRQ flags, the kernel
> could still panic. I understand this is not a "normal" configuration
> but, may be we could make the panic option based on whether
> we actually use the system regs early enough ?

I see, however I'm not sure what happens in the GIC drivers if we have a 
CPU running with a GICv3 and other CPUs with something else... But of 
course this is not technically limited by the arm64 capabilities handling.

What behaviour would you be looking for? A way to prevent the CPU to be 
brought up instead of panicking?

> Btw, I am rewriting the capabilities infrastructure to allow per-cap
> control on how it should be treated. I might add an EARLY scope for
> caps which could cover this and may be VHE.

Thanks,

--
Julien Thierry