v2:
- Replace the hostmem patch to use qemu_fd_getfs() instead [David]
- Define enum instead of using string for fs type [Dan]
- One more patch added to deliver error to QMP response (rather than
  keeping some of it in STDERR), so the response should contain richer
  information and looks prettier.

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: Postcopy is not supported: Host backend files need to be TMPFS or HUGETLBFS only

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

{ "execute": "migrate-set-capabilities" , "arguments": { "capabilities": [ { "capability": "postcopy-ram", "state": true } ] } }
{"error": {"class": "GenericError", "desc": "Postcopy is not supported: Host backend files need to be TMPFS or HUGETLBFS only"}}

Please have a look, thanks.

Peter Xu (4):
  util/mmap-alloc: qemu_fd_getfs()
  vl.c: Create late backends before migration object
  migration/postcopy: Detect file system on dest host
  migration: Allow postcopy_ram_supported_by_host() to report err

 include/qemu/mmap-alloc.h |  7 +++
 migration/migration.c     |  9 ++--
 migration/postcopy-ram.c  | 89 +++++++++++++++++++++++++++------------
 migration/postcopy-ram.h  |  3 +-
 migration/savevm.c        |  3 +-
 softmmu/vl.c              |  9 +++-
 util/mmap-alloc.c         | 28 ++++++++++++
 7 files changed, 111 insertions(+), 37 deletions(-)

-- 
2.39.1