[RFC PATCH v3 3/3] arm64/kernel: enable A53 erratum #8434319 handling at runtime

[will.deacon@arm.com (Will Deacon)]

On Mon, Mar 05, 2018 at 05:29:26PM +0000, Ard Biesheuvel wrote:
> On 5 March 2018 at 17:22, Will Deacon <will.deacon@arm.com> wrote:
> > On Wed, Feb 14, 2018 at 11:36:45AM +0000, Ard Biesheuvel wrote:
> >> Omit patching of ADRP instruction at module load time if the current
> >> CPUs are not susceptible to the erratum.
> >>
> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> >> ---
> >> Open question: how should we handle big.LITTLE configurations where affected
> >> Cortex-A53s may appear late.
> >
> > We should fail to bring them online. I think the infrastructure already
> > exists for this and we used it for other errata already.
> >
> 
> I think that is what it does currently. The question is whether that
> should be considered a regression or not.

This can only happen with maxcpus=, right? I wouldn't be too worried about
it in that case.

> 
> >> diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
> >> index bb263820de13..39134c46bb13 100644
> >> --- a/arch/arm64/include/asm/cpucaps.h
> >> +++ b/arch/arm64/include/asm/cpucaps.h
> >> @@ -45,7 +45,8 @@
> >>  #define ARM64_HARDEN_BRANCH_PREDICTOR                24
> >>  #define ARM64_HARDEN_BP_POST_GUEST_EXIT              25
> >>  #define ARM64_HAS_RAS_EXTN                   26
> >> +#define ARM64_WORKAROUND_843419                      27
> >>
> >> -#define ARM64_NCAPS                          27
> >> +#define ARM64_NCAPS                          28
> >>
> >>  #endif /* __ASM_CPUCAPS_H */
> >> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> >> index 07823595b7f0..c065d5649b1b 100644
> >> --- a/arch/arm64/kernel/cpu_errata.c
> >> +++ b/arch/arm64/kernel/cpu_errata.c
> >> @@ -228,6 +228,23 @@ static int qcom_enable_link_stack_sanitization(void *data)
> >>  }
> >>  #endif       /* CONFIG_HARDEN_BRANCH_PREDICTOR */
> >>
> >> +static bool __maybe_unused
> >> +needs_erratum_843419_workaround(const struct arm64_cpu_capabilities *entry,
> >> +                             int scope)
> >> +{
> >> +     u32 cpuid = read_cpuid_id();
> >> +
> >> +     WARN_ON(scope != SCOPE_LOCAL_CPU || preemptible());
> >> +
> >> +     if ((cpuid & MIDR_CPU_MODEL_MASK) != MIDR_CORTEX_A53)
> >> +             return false;
> >> +     else if ((cpuid & (MIDR_REVISION_MASK | MIDR_VARIANT_MASK)) == 0x4)
> >> +             /* erratum was fixed in some versions of r0p4 */
> >> +             return !(read_cpuid(REVIDR_EL1) & BIT(8));
> >
> > The rXpY information is in the MIDR, so this is checking for something else
> > afaict.
> >
> 
> No, it checks the REVIDR of r0p4 parts, of which bit 8 tells us if
> this specific erratum has been fixed.

/me checks errata notice.

Ok, fair enough! Given that it looks like this mechanism is used for other
errata too, it would make sense to support it in the arm64_cpu_capabilities
structure alongside the midr stuff.

Will