Postcopy can fail in a weird way when guest mem is put onto a random file:

https://bugzilla.redhat.com/show_bug.cgi?id=2057267

It's because we only check userfault privilege on dest QEMU but don't check
memory types.  We do so only until the UFFDIO_REGISTER right after we
switch to postcopy live migration from precopy but it could be too late.

This series tries to make it fail early by checking ramblock fs type if
backed by a memory-backend-file.

Now when it happens it'll fail the dest QEMU from the start:

./qemu-system-x86_64 \
        -global migration.x-postcopy-ram=on \
        -incoming defer \
        -object memory-backend-file,id=mem,size=128M,mem-path=$memfile \
        -machine memory-backend=mem

qemu-system-x86_64: Host backend files need to be TMPFS or HUGETLBFS only
qemu-system-x86_64: Postcopy is not supported

It will also fail e.g. QMP migrate-set-capabilities properly.

Please have a look, thanks.

Peter Xu (3):
  hostmem: Detect and cache fs type for file hostmem
  vl.c: Create late backends before migration object
  migration/postcopy: Detect file system on dest host

 backends/hostmem-file.c  | 37 ++++++++++++++++++++++++++++++++++++-
 include/sysemu/hostmem.h |  1 +
 migration/postcopy-ram.c | 28 ++++++++++++++++++++++++----
 softmmu/vl.c             |  9 +++++++--
 4 files changed, 68 insertions(+), 7 deletions(-)

-- 
2.39.1