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

[Aneesh Kumar K.V]

On Fri, 7 Oct 2011 10:27:56 +0100, "Daniel P. Berrange" <address@hidden> wrote:
> On Thu, Sep 29, 2011 at 04:22:16PM +0100, Daniel P. Berrange wrote:
> > 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 ?
> 
> Does anyone have an clear answer for this ?
> 
> I've done some tests with ever larger mount tags, and managed to increase
> the MAX_TAG_LEN value to 1023  before I started getting guest failures.
> 
> So if the config space is really 1023 bytes in size, it doesn't seem too
> unrealistic to allow 255 bytes of it for the mount_tag, or at the very
> least increase it from 32 to 128 ?
> 

Last time we discussed this Anthony wanted to keep the config space
usage minimal, hence we agreed on the size 32 bytes. 

Anthony,

Any comments ?

-aneesh