[RFC] provide an API to userspace doing memory snapshot

[Wenchao Xia <wenchaolinuxkvm@gmail.com>]

Hi,
  I'd like to add/export an function which allow userspace program
to take snapshot for a region of memory. Since it is not implemented yet
I will describe it as C APIs, it is quite simple now and if it is worthy
I'll improve the interface later:

Simple prototype:
C API in userspace:
/*
 *   This function will mark a section of memory as COW, and return
 * a new virtual address of it. User space program can dump out the
 * content as a snapshot while other thread continue modify the content
 * in the region.
 *   @addr: the virtual address to be snapshotted.
 *   @length: the length of it.
 *   This function returns a new virtual address which can be used as
 * snapshot. Return NULL on fail.
 */
void *memory_snapshot_create(void *addr, uint64_t length);

/*
 *   This function will free the memory snapshot.
 *   @addr: the virtual snapshot addr to be freed, it should be the
 * returned one in memory_snapshot_create().
 */
void memory_snapshot_delete(void *addr);

In kernel space:
  The pages in those virtual address will be marked as COW. Take a
page with physical addr P0 as example, it will have two virtual addr:
old A0 and new A1. When modified, kernel should create a new page P1
with same contents, and mapping A1 to P1. When NUMA is used, P1 can
be a slower page.
  It is quite like fork(), but only COW part of pages. Maybe add it
as an ioctl() in kvm.ko, and change the input/output as a structure
describe guest memory state.

Why bring it to kernel space:
  Compared with fork():
  1 take less RAM, less halt time by avoid marking unnecessary pages
COW.
  2 take less RAM if API can return a bitmap let qemu consume dirty
page first.
  3 much nicer userspace program model, no need to pipe or memcpy(),
brings better performance.
  4 optimization space in kernel space, since snapshoted pages
can be set into slower memory in NUMA when change comes.