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

[Daniel P. Berrange]

On Tue, Nov 01, 2011 at 01:27:56PM -0500, Anthony Liguori wrote:
> On 10/20/2011 10:13 AM, Daniel P. Berrange wrote:
> >On Fri, Oct 07, 2011 at 04:49:13PM +0530, Aneesh Kumar K.V wrote:
> >>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:
> >>>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.
> >
> >Ping ? Anyone ....
> >
> >Does anyone have any clear information about the per-device config
> >space we have available ?  As above I'd really like us to raise
> >the mount_tag length even just a little bit higher for QEMU 1.0,
> >if we have the PCI config space available to play with.
> 
> Yes, PCI PIO space is very small.  I think 128 is even pushing it.

Odd, because I managed to pass through a 1023 byte path without
appearing to have any trouble. Is the space per-device, or global
to all devices. If the latter, I could understand the desire to
keep it smaller.

> Why not add a feature that exchanges the tag through another
> mechanism such that there doesn't need to be a limit?  It could be
> as simple as adding an fsstat .L operation or something like that.

That would require kernel side updates too I presume, so if that
kind of change is the only option, I think I'll just have to change
my app's code to cope with the current smaller limits for now.

Regards,
Daniel
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