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

[Robin Murphy]

On 2022-09-09 17:59, James Morse wrote:
> Hello!
> 
> 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?

Robin.

> This won't stop user-space from trying to use the instructions to see
> if they work - but such software is already broken on big/little systems
> with mismathed features.
> 
> Removing the BF16 feature involves removing both the HWCAP, as was done
> by commit 44b3834b2eed "arm64: errata: Remove AES hwcap for COMPAT tasks",
> and the emulated view of that register that user-space has.
> (This wasn't previously needed as aarch32 ID registers aren't accessible
> like this).
> 
> As there are now two of these things, this series tries to add a more
> maintainable way to remove features, to avoid spilling workaround like
> this into cpufeature.c.
> 
> Instead, cpu_errata.c modifies the user_mask that is used for emulation
> of the id registers, and cpufeature.c builds the HWCAPs based on this.
> 
> I have patches to convert the AES workaround to do the same, but that
> should wait until the aarch32 ID registers are generated by the sysreg
> awk scripts (it needs some new masks defined).
> 
> [0] https://developer.arm.com/documentation/SDEN1873351/1400/?lang=en
> 
> Thanks,
> 
> James Morse (3):
>    arm64: cpufeature: Force HWCAP to be based on the sysreg visible to
>      user-space
>    arm64: cpufeature: Expose get_arm64_ftr_reg() outside cpufeature.c
>    arm64: errata: remove BF16 HWCAP due to incorrect result on
>      Cortex-A510
> 
>   Documentation/arm64/silicon-errata.rst |  2 ++
>   arch/arm64/Kconfig                     | 14 +++++++++
>   arch/arm64/include/asm/cpufeature.h    |  2 ++
>   arch/arm64/kernel/cpu_errata.c         | 27 ++++++++++++++++
>   arch/arm64/kernel/cpufeature.c         | 43 ++++++++++++++++++++------
>   arch/arm64/tools/cpucaps               |  1 +
>   6 files changed, 79 insertions(+), 10 deletions(-)
>