[PATCH 0/3] arm64: errata: remove BF16 HWCAP due to incorrect result on Cortex-A510

[James Morse]

Hi Robin,

On 09/09/2022 19:29, Robin Murphy wrote:
> On 2022-09-09 17:59, James Morse wrote:
>> Cortex-A510 has an erratum where the BFMMLA or VMMLA instructions might
>> produce the incorrect result. This only happens if two Cortex-A510 CPUs
>> are configured by the SoC manafacturer in a particular way to share some
>> hardware, and are using it at the same time. It isn't possible for linux
>> to know how the CPUs were configured, so the only option is to disable
>> the BF16 feature for all Cortex-A510 CPUs.

> Hmm, the TRM doesn't seem to describe any obvious restrictions on accessing IMP_CPUCFR_EL1
> - is there some other nefarious secret at play there?

HCR_EL2.TIDCP.

KVM traps all IMP_* registers from EL1, and has no ability to emulate them.
For an erratum workaround at EL1, touching an imp-def register is the wrong thing to do.

We could add some discovery API with firmware to determine if the part is affected, but I
think that is overkill for this. I'm pretty sure affected Cortex-A510 silicon is only in
mobile phones, which can have the Kconfig option disabled if the target platform isn't
affected. (Even with GKI, I assume things don't run one kernel image in the same way
distros do)


Thanks,

James