[PATCH v14 04/39] arm64/sme: Provide ABI documentation for SME

[Catalin Marinas]

On Tue, Apr 19, 2022 at 12:22:12PM +0100, Mark Brown wrote:
> +* There are a number of optional SME features, presence of these is reported
> +  through AT_HWCAP2 through:
> +
> +	HWCAP2_SME_I16I64
> +	HWCAP2_SME_F64F64
> +	HWCAP2_SME_I8I32
> +	HWCAP2_SME_F16F32
> +	HWCAP2_SME_B16F32
> +	HWCAP2_SME_F32F32
> +	HWCAP2_SME_FA64

Marc pointed out that in combination with FEAT_WFxT, we used all the
HWCAP2 bits (32). While we are ok for now, we'll soon need to look into
what to do when the next features turn up. Some options:

1. Only provide HWCAP2_SME and let the ID_AA64SMFR0_EL1 features be
   probed via MRS emulation. It doesn't solve the problem but it buys us
   a bit of time.

2. Don't bother with any new HWCAPs, just rely on MRS emulation (we have
   HWCAP_CPUID advertising this).

3. Start using the upper 32-bit of HWCAP and HWCAP2 (we initially didn't
   go into these as there was a slight chance of merging ILP32). Does
   the libc rely on the upper bits for anything? Or does it just assume
   a 32-bit HWCAPs layout?

4. Introduce HWCAP3.

Szabolcs, any thoughts?

Thanks.

-- 
Catalin