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Re: [Qemu-devel] [PATCH] Raise 9pfs mount_tag limit from 32 to 255 bytes

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [PATCH] Raise 9pfs mount_tag limit from 32 to 255 bytes
Date: Thu, 29 Sep 2011 16:22:16 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Thu, Sep 29, 2011 at 08:23:49PM +0530, Aneesh Kumar K.V wrote:
> On Thu, 29 Sep 2011 11:34:21 +0100, "Daniel P. Berrange" <address@hidden> 
> wrote:
> > From: "Daniel P. Berrange" <address@hidden>
> > 
> > The Linux guest kernel does not appear to have a problem handling
> > a mount_tag larger than 32 bytes. Increase the limit to 255 bytes,
> > though perhaps it can be made larger still, or not limited at all ?
> > 
> > Tested with a 3.0.4 kernel and a mount_tag 255 bytes in length.
> > 
> > * hw/9pfs/virtio-9p.h: Change MAX_TAG_LEN to 255
> mount_tag is passed via pci config space, do we want to have 255 bytes
> out of that for device identification.

How big is the config space available for each 9pfs device and what
other info does it need to keep there ?

>                                        Is this change driven by 
> the virt-manager UI where we specify source and target directory for
> file system ?. I would like to understand how that target
> directory is used in case of other user like NFS ?. If this is something
> used to get automout (I am not sure how we drive such an automount) of
> those mounts points, then it should be ok for VirtFS to generate mount tag 
> internally
> which will fit 32 byte limit and use the target directory for
> automounting the mount point using the internally generated mount tag

There's no virt-manager involved in what I'm working on. It is a tiny
app with KVM embedded behind the scenes. All that runs inside the virtual
machine is a minature 'init' process which mounts any 9pfs filesystems
it finds, creates some device nodes, and then runs the application bianry.
The 'init' code does not have any knowledge of where the 9pfs filesystems
need to be mounted, that info must come from the host, because it can
vary on each boot.

I don't want to have to pass separate 'mount_tag'; -> 'mount point'
mapping data through to the guest via a side-channel. I just want to
put the target mount point path, directly in the mount tag.

Originally I was trying to namespace my mount tags


but this hit the 32 byte limit almost immediately, so now I just
put the raw '/mount/point/path' in each mount tag.

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