Allow ddb and dtrace use the DMAP region on arm64

[andrew]

Summary

When writing to memory on arm64 we may be trying to be accessing a
read-only page. In this case try to access via the DMAP region to
get a writable location.

While here simplify writing data in DDB and stop trashing the size as
it is passed into the cache handling functions.

[mhorne]

Why should the DMAP alias be writable? Its protections are supposed to be kept in sync with the corresponding KVA page. You add a comment to this effect in D32026, but I think pmap_change_props_locked() is still missing the code that actually updates the DMAP protections (at least I don't see it).

[andrew]

• Split out the translate code inot a new function
• Add dtrace support

[andrew]

In D32053#723298, @mhorne wrote:

Why should the DMAP alias be writable? Its protections are supposed to be kept in sync with the corresponding KVA page. You add a comment to this effect in D32026, but I think pmap_change_props_locked() is still missing the code that actually updates the DMAP protections (at least I don't see it).

In the current version of that change I forced the DMAP to be writable. I expect to change it to match the comment when I have time to update arm64_get_writable_addr to map the memory as writable.