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

[Anthony Liguori <aliguori@us.ibm.com>]

On 11/01/2011 01:48 PM, Daniel P. Berrange wrote:
> 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"<berrange@redhat.com>   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.

It's global so it ends up being a problem when you have many devices.

>
>> 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.

Right.

Regards,

Anthony Liguori

>
> Regards,
> Daniel