Re: [PATCH 13/16] arm64: Add support for checking errata based on a list of MIDRS

[Suzuki K Poulose]

On 26/01/18 14:16, Dave Martin wrote:
> On Tue, Jan 23, 2018 at 12:28:06PM +0000, Suzuki K Poulose wrote:
> > Add helpers for detecting an errata on list of midr ranges
> > of affected CPUs.
>
> This doesn't describe what the patch does: instead, helpers are being
> added for checking whether an MIDR falls in one of multiple affected
> model(s) and or revision(s).
>
> Doing this makes sense, but is it really worth it?

Well, we need th MIDR list helpers anyway for other things:
  - White list of CPUs where we know KPTI is not needed
  - Black list of CPUs where DBM shouldn't be enabled.

So all we do is add a new type which could reduce the number of entries.

> We might save 100-200 bytes in the kernel image for now, but a common
> workaround for errata on multiple unrelated cpus is surely a rare case.
>
> Only if there are many such lists, or if the lists become large does
> this start to seem a clear win.
>
> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> > ---
> >   arch/arm64/include/asm/cpufeature.h |  1 +
> >   arch/arm64/kernel/cpu_errata.c      | 40 ++++++++++++++++++++++---------------
> >   2 files changed, 25 insertions(+), 16 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> > index a3d54c2c411f..70712de687c7 100644
> > --- a/arch/arm64/include/asm/cpufeature.h
> > +++ b/arch/arm64/include/asm/cpufeature.h
>
> [...]
>
> > @@ -330,22 +353,7 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
> >   #ifdef CONFIG_HARDEN_BRANCH_PREDICTOR
> >   	{
> >   		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
> > -		ERRATA_MIDR_ALL_VERSIONS(MIDR_CORTEX_A57),
> > -		.enable = enable_psci_bp_hardening,
> > -	},
> > -	{
> > -		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
> > -		ERRATA_MIDR_ALL_VERSIONS(MIDR_CORTEX_A72),
> > -		.enable = enable_psci_bp_hardening,
> > -	},
> > -	{
> > -		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
> > -		ERRATA_MIDR_ALL_VERSIONS(MIDR_CORTEX_A73),
> > -		.enable = enable_psci_bp_hardening,
> > -	},
> > -	{
> > -		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
> > -		ERRATA_MIDR_ALL_VERSIONS(MIDR_CORTEX_A75),
> > +		ERRATA_MIDR_RANGE_LIST(cortex_bp_harden_cpus),
>
> Could we just use a macro to generate multiple structs, instead of
> inventing a new type of struct?

We could. Somehow, I don't think we are over engineering much here.

Cheers
Suzuki