devices: pmem: implement flush using msync()

Previously, writable pmem devices implemented the flush command using
fsync(); however, this does not guarantee synchronization of memory
mappings via mmap() to the file on disk.  What we actually need is
msync() on the pmem file mapping, but we don't have access to that
mapping in the pmem child process, and it isn't trivial to pass it along
since it is owned by the Vm object once it has been added as a
mmap_arena.

In order to call msync() on the mapping, add a new VmControl socket so
that the pmem device can request that the main process issues an msync()
on the MemoryMappingArena identified by its slot number.

BUG=chromium:1007535
TEST=mount filesystem on /dev/pmem0 and sync; verify msync in strace

Change-Id: Id0484757c422cf81d454fd54012a12dbcc1baaf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2044365
Reviewed-by: Stephen Barber <smbarber@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Daniel Verkamp <dverkamp@chromium.org>

0 files changed, 0 insertions, 0 deletions