Single memory slot

[Avi Kivity <avi@redhat.com>]

One way to improve the gfn_to_pfn() memslot search is to register just 
one slot.  This can only work on 64-bit, since even the smallest guests 
need 4GB of physical address space.  Apart from speeding up 
gfn_to_page(), it would also speed up mmio which must iterate over all 
slots, so a lookup cache cannot help.

This would require quite a bunch of changes:
- modify gfn_to_pfn() to fail gracefully if the page is in the slot but 
unmapped (hole handling)
- modify qemu to reserve the guest physical address space
- modify qemu memory allocation to use MAP_FIXED to allocate memory
- some hack for the vga aliases (mmap an fd multiple times?)
- some hack for the vmx-specific pages (e.g. APIC-access page)

Not sure it's worthwhile, but something to keep in mind if a simple 
cache or sort by size is insufficient due to mmio.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.