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

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

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:
>>>> 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"<berrange@redhat.com>  wrote:
>>>>>> From: "Daniel P. Berrange"<berrange@redhat.com>
>>>>>>
>>>>>> 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.
>
> 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.

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.

Regards,

Anthony Liguori

>
> Regards,
> Daniel