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

[Vivek Goyal <vgoyal@redhat.com>]

On Sat, Dec 22, 2018 at 05:27:28PM +0800, jiangyiwen wrote:
> On 2018/12/11 1:31, Dr. David Alan Gilbert (git) wrote:
> > 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
> > 
> 
> Hi Dave,
> 
> I encounter a problem after running qemu with virtio-fs,
> 
> I find I only can mount virtio-fs using the following command:
> mount -t virtio_fs /dev/null /mnt/virtio_fs/ -o tag=myfs,rootmode=040000,user_id=0,group_id=0
> or mount -t virtio_fs /dev/null /mnt/virtio_fs/ -o tag=myfs,rootmode=040000,user_id=0,group_id=0,dax
> 
> Then, I want to know how to use "cache=always" or "cache=none", even "cache=auto", "cache=writeback"?
> 
> Thanks,
> Yiwen.

Hi Yiwen,

As of now, cache options are libfuse daemon options. So while starting
daemon, specify "-o cache=none" or "-o cache=always" etc. One can not
specify caching option at virtio-fs mount time.

Thanks
Vivek