Use ATTR_DBM for ATTR_SW_DBM instead of a software bit.

[markj]

Summary

On ARMv8.0 ATTR_DBM is reserved. But Linux uses it anyway, so I presume
it is safe to do so in FreeBSD as well.

I am torn on whether to keep using "ATTR_SW_DBM" or whether I should
just replace all references with ATTR_SW_DBM. Any preferences?

[alc]

Do we have access to a machine with hardware DBM support?

[markj]

In D21121#458634, @alc wrote:

Do we have access to a machine with hardware DBM support?

Not yet. It looks like the thunderx2 does not support hardware DBM or AF management: http://dmesgd.nycbug.org/index.cgi?do=view&id=4798

[alc]

I am torn on whether to keep using "ATTR_SW_DBM" or whether I should
just replace all references with ATTR_SW_DBM. Any preferences?

No, not really.

[andrew]

In D21121#458634, @alc wrote:

Do we have access to a machine with hardware DBM support?

As far as I know only in the form of a simulator, e.g. the Architecture Envelope Model from https://developer.arm.com/tools-and-software/simulation-models/fixed-virtual-platforms