bhyve: remove empty E820 entries

[corvink]

Summary

When reserving a block with the same size of a RAM segement, we can end up with
an empty RAM segmenet. Avoid that by removing this empty segment from the E820
table.

[corvink]

@markj @jhb Any feedback?

[jhb]

usr.sbin/bhyve/amd64/e820.c

241

244

I suggest making the code match the comments by tweaking it as so:

TAILQ_INSERT_BEFORE(element, new_element, chain);
element->base = end;

/*
  * (New comment here)
  */
if (end == element->end) {
   TAILQ_REMOVE(...);
    free(element);
}

Or even alternatively, make this a new top-level case:

if (base == element->base && end == element->end) {
    /* The new entry replaces an existing entry. */
    TAILQ_INSERT_BEFORE(element, new_element, chain);
    TAILQ_REMOVE(&e820_table, element, chain);
    free(element);
} else if (base == element->base) {
    /* Existing code */
} else if (end == element->end) {
    ...
} else {
    ...
}

[corvink]

• use a new top level case to detect empty entries