sys_util: add MemoryMappingArena

There is a hard-limit to the number of MemoryMaps that can be added to a
KVM VM, a arch-dependent number defined as KVM_USER_MEM_SLOTS. e.g: on
x86 this is 509 (512 - 3 internal slots).

For most purposes, this isn't too much of an issue, but there are some
cases where one might want to share a lot of mmaps with a Guest. e.g:
virtio-fs uses a large cache region for mapping in slices of file fds
directly into guest memory. If one tries to add a new KVM memory region
for each mmap, the number of available slots is quickly exhausted.

MemoryMappingArena is a way to work around this limitation by allocating
a single KVM memory region for a large slice of memory, and then using
mmap with MAP_FIXED to override slices of this "arena" hostside, thereby
achieving the same effect without quickly exhausting the number of KVM
memory region slots.

BUG=chromium:936567
TEST=cargo test -p sys_util

Change-Id: I89cc3b22cdba6756b2d76689176d7147cf238f07
Reviewed-on: https://chromium-review.googlesource.com/1546600
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Zach Reizner <zachr@chromium.org>

0 files changed, 0 insertions, 0 deletions