illumos gate

A/D tracking (and thus the better interfaces) have been a focus for work done in Propolis, so that has been one of the primary test cases for this code, where platform bits featuring this change, in addition to others, were used as the primary Propolis development and test environment. That also exposed it to more varied usage in terms of VM ioctls and the segment driver, as compared to "vanilla c-bhyve". The bulk of that testing was done with the gpt_track_dirty variable set to its default of false. Some spot-checks of that functionality have been done, and it was observed working, but more heavy testing will be done as more interfaces for migration are built into Propolis. It will remain default-off for the time being, as such tracking of dirty pages does come at a performance cost, and it would be silly to do it when not required.

In addition to the Propolis tests, I ran these bits on both Intel and AMD machines, spinning up the typical battery of bhyve guests for smoke-testing. Windows, Linux, illumos, and OpenBSD guests booted, rebooted, and ran as expected.

The ticket for #13981 includes a test program used to exercise the edge case of unmapping an object from the guest-physical address space of a VM which itself is mapped into a userspace process of the host. This is expected to incur a SIGSEGV in that host process, should it attempt to access the now-unmapped guest memory, rather than cause a problem (read: panic) in seg_vmm. Running it under the test bits, it does segfault safely, as expected.

I created #14251 to address the need for creating a real interface (beyond mdb -kw) for enabling dirty page tracking on bhyve instances.