bhyve: Avoid triggering false -Wfree-nonheap-object warnings.

[jhb]

Summary

XHCI port and slot numbers are 1-based rather than 0-based. To handle
this, bhyve was subtracting one item from the pointers saved in the
softc so that index 1 accessed index 0 of the allocated array.

However, this is UB and confused GCC 12. The compiler noticed that
the calls to free() were using an offset and emitted a warning.
Rather than storing UB pointers in the softc, push the decrement
operation into the existing macros that wrap accesses to the relevant
arrays.

[markj]

Can we avoid the UB by simply allocating an extra slot in each array? This trick is saving 16 bytes of memory per xhci instance...

[jhb]

In D36829#836024, @markj wrote:

Can we avoid the UB by simply allocating an extra slot in each array? This trick is saving 16 bytes of memory per xhci instance...

Well, this is even safe on CHERI (too much C uses tricks like this). But we could allocate an extra one perhaps and just leave index 0 unused. We could also perhaps just adjust the references, especially if they are all wrapped via macros like XHCI_DEVINST_PTR() which could deal with subtracting one from the index.

[jhb]

Adjust indices on use

[markj]

usr.sbin/bhyve/pci_xhci.c

294

These should technically have parens around the expansion.

2833–2834

We're still using the old trick for portregs - probably it's better to be consistent one way or the other?

[jhb]

usr.sbin/bhyve/pci_xhci.c

2833–2834

Oh, yes, agreed.

[jhb]

Fix portregs as well and add parens