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

[jiangyiwen]

On 2018/12/27 3:08, Vivek Goyal wrote:
> 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" <address@hidden>
>>>
>>> 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
> 
> .
> 

Ok, I get it, thanks.

Yiwen.