large page support for kvm

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

The npt patches started me thinking on large page support (2MB/4MB 
pages), and I think we can implement them even when npt/ept are not 
available.

Here's a rough sketch of my proposal:

- For every memory slot, allocate an array containing one int for every 
potential large page included within that memory slot.  Each entry in 
the array contains the number of write-protected 4KB pages within the 
large page frame corresponding to that entry.

For example, if we have a memory slot for gpas 1MB-1GB, we'd have an 
array of size 511, corresponding to the 511 2MB pages from 2MB upwards.  
If we shadow a pagetable at address 4MB+8KB, we'd increment the entry 
corresponding to the large page at 4MB.  When we unshadow that page, 
decrement the entry.

- If we attempt to shadow a large page (either a guest pse pte, or a 
real-mode pseudo pte), we check if the host page is a large page.  If 
so, we also check the write-protect count array.  If the result is zero, 
we create a shadow pse pte.

- Whenever we write-protect a page, also zap any large-page mappings for 
that page.  This means rmap will need some extension to handle pde rmaps 
in addition to pte rmaps.

- qemu is extended to have a command-line option to use large pages to 
back guest memory.

Large pages should improve performance significantly, both with 
traditional shadow and npt/ept.  Hopefully we will have transparent 
Linux support for them one day.

-- 
error compiling committee.c: too many arguments to function


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