[Qemu-devel] [RFC PATCH 0/7] virtio-fs: shared file system for virtual machines3

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

Hi,
  This is the first RFC for the QEMU side of 'virtio-fs';
a new mechanism for mounting host directories into the guest
in a fast, consistent and secure manner.  Our primary use
case is kata containers, but it should be usable in other scenarios
as well.

There are corresponding patches being posted to Linux kernel,
libfuse and kata lists.

For a fuller design description, and benchmark numbers, please see
Vivek's posting of the kernel set here:

https://marc.info/?l=linux-kernel&m=154446243024251&w=2

We've got a small website with instructions on how to use it, here:

https://virtio-fs.gitlab.io/

and all the code is available on gitlab at:

https://gitlab.com/virtio-fs

QEMU's changes
--------------

The QEMU changes are pretty small; 

There's a new vhost-user device, which is used to carry a stream of
FUSE messages to an external daemon that actually performs
all the file IO.  The FUSE daemon is an external process in order to
achieve better isolation for security and resource control (e.g. number
of file descriptors) and also because it's cleaner than trying to
integrate libfuse into QEMU.

This device has an extra BAR that contains (up to) 3 regions:

 a) a DAX mapping range ('the cache') - into which QEMU mmap's
    files on behalf of the external daemon; those files are
    then directly mapped by the guest in a way similar to a DAX
    backed file system;  one advantage of this is that multiple
    guests all accessing the same files should all be sharing
    those pages of host cache.

 b) An experimental set of mappings for use by a metadata versioning
    daemon;  this mapping is shared between multiple guests and
    the daemon, but only contains a set of version counters that
    allow a guest to quickly tell if its metadata is stale.

TODO
----

This is the first RFC, we know we have a bunch of things to clear up:

  a) The virtio device specificiation is still in flux and is expected
     to change

  b) We'd like to find ways of reducing the map/unmap latency for DAX

  c) The metadata versioning scheme needs to settle out.

  d) mmap'ing host files has some interesting side effects; for example
     if the file gets truncated by the host and then the guest accesses
     the mapping, KVM can fail the guest hard.

Dr. David Alan Gilbert (6):
  virtio: Add shared memory capability
  virtio-fs: Add cache BAR
  virtio-fs: Add vhost-user slave commands for mapping
  virtio-fs: Fill in slave commands for mapping
  virtio-fs: Allow mapping of meta data version table
  virtio-fs: Allow mapping of journal

Stefan Hajnoczi (1):
  virtio: add vhost-user-fs-pci device

 configure                                   |  10 +
 contrib/libvhost-user/libvhost-user.h       |   3 +
 docs/interop/vhost-user.txt                 |  35 ++
 hw/virtio/Makefile.objs                     |   1 +
 hw/virtio/vhost-user-fs.c                   | 517 ++++++++++++++++++++
 hw/virtio/vhost-user.c                      |  16 +
 hw/virtio/virtio-pci.c                      | 115 +++++
 hw/virtio/virtio-pci.h                      |  19 +
 include/hw/pci/pci.h                        |   1 +
 include/hw/virtio/vhost-user-fs.h           |  79 +++
 include/standard-headers/linux/virtio_fs.h  |  48 ++
 include/standard-headers/linux/virtio_ids.h |   1 +
 include/standard-headers/linux/virtio_pci.h |   9 +
 13 files changed, 854 insertions(+)
 create mode 100644 hw/virtio/vhost-user-fs.c
 create mode 100644 include/hw/virtio/vhost-user-fs.h
 create mode 100644 include/standard-headers/linux/virtio_fs.h

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2.19.2