Re: [Qemu-devel] last one question about VirtFS

[Greg Kurz]

On Wed, 27 Mar 2019 16:22:33 +0200
"П, Алексей" <address@hidden> wrote:

> Please excuse any errors on my part and for my insistense, i had closed my
> research in case of "handle" backend, the last case which i cannot resolve
> is to find purpose of "synthetic 9p file system". I'm stuck in testing with
> command below:
> 
> 
> 
> qemu-system-x86_64 -cpu max \
>                    -accel kvm \
>                    -m size=512M \
>                    -blockdev driver=file,node-name=hdd,filename=debian.img \
>                    -device driver=ide-hd,drive=hdd \
>                    -virtfs_synth
> 
> I mean i cannot find any example of using command mentioned above.
> 
> Please provide me If there are any material to study.
> 
> Thanks.

The original purpose of the synth backend is best explained in the
changelog of the patch that introduced it:

commit 9db221ae73a18e0bd2b1ee6c7dc1904ed06fb464
Author: Aneesh Kumar K.V <address@hidden>
Date:   Tue Oct 25 12:10:40 2011 +0530

    hw/9pfs: Add synthetic file system support using 9p
    
    This patch create a synthetic file system with mount tag
    v_synth when -virtfs_synth command line option is specified
    in qemu. The synthetic file system can be mounted in guest
    using 9p using the below command line
    
    mount -t 9p -oversion=9p2000.L,trans=virtio v_synth  <mountpint>
    
    Synthetic file system enabled different qemu subsystem to register
    callbacks for read and write events from guest. The subsystem
    can create directories and files in the synthetic file system as show
    in ex below
    
        qemu_v9fs_synth_mkdir(NULL, 0777, "test2", &node);
        qemu_v9fs_synth_add_file(node, 0777, "testfile",
                                 my_test_read, NULL, NULL);
    
    Signed-off-by: Aneesh Kumar K.V <address@hidden>


This never took momentum really and no QEMU subsystem ever registered
anything.

I finally found some use for it in order to test the core 9p
code with qtest (see 2893ddd5988a3).

Cheers,

--
Greg