arm64: export new HWCAP features

[mhorne]

Summary

This includes the addition of elf_hwcap2 into the sysvec, and a separate
function to parse for those features.

This only exposes features which require no further configuration, e.g.
indicating the presence of certain instructions. Larger features (SVE)
will not be advertised until we actually support them.

Reference: https://www.kernel.org/doc/html/v5.8/arm64/elf_hwcaps.html

[markj]

sys/arm64/arm64/identcpu.c

1148 ↗

(On Diff #75697)

Suppose CPUs 0 and 1 have different capabilities: CPU 0 has no capabilities covered by the elf_hwcap2 mask, but CPU 1 has at least one. Then, when cpu == 1 we will see that elf_hwcap2 == 0 and set elf_hwcap2 to the set of CPU 1's capabilities even though they are not shared by all CPUs.

I suppose this could happen with elf_hwcap too, but it seems less likely that a CPU has no capabilities in that set.

[mhorne]

sys/arm64/arm64/identcpu.c

1148 ↗

(On Diff #75697)

That's a good point. I think instead we could initialize elf_hwcap2 to (u_long)-1 and use &= all the way.

[andrew]

Do any of the new caps require kernel support before they can be used? If so we shouldn't report them to the user until that support is in place.

sys/arm64/arm64/identcpu.c

1148 ↗

(On Diff #75697)

I think we should be using user_cpu_desc to set both elf_hwcap and elf_hwcap2. This will ensure we are consistent with what we report through both methods.

[mhorne]

Update post D26069.

[mhorne]

In D26031#577687, @andrew wrote:

Do any of the new caps require kernel support before they can be used? If so we shouldn't report them to the user until that support is in place.

Not as far as I know, most simply indicate the presence of instructions. I need to do a second reading to be sure before committing.

[mhorne]

Expose SSBS in user_cpu_desc. Annotate elf_hwcap and elf_hwcap2 as __read_frequently.

[mhorne]

In D26031#578624, @mhorne wrote:

In D26031#577687, @andrew wrote:

Do any of the new caps require kernel support before they can be used? If so we shouldn't report them to the user until that support is in place.

Not as far as I know, most simply indicate the presence of instructions. I need to do a second reading to be sure before committing.

ARMv8.5-BTI will require some work in the kernel before it can be exposed to userspace, so it is still gated behind user_cpu_desc. The rest of these features are safe to expose.