Xen devel - Sep 2009 - [PATCH] PoD: Check p2m assumption in debug builds

The PoD code assumes that if:
* A page is in a domain''s p2m table
* And it''s owned by the domain
* And it''s not a xenheap page
then:
* It''s on the domain''s page list.

This patch adds a check for this assumption when debug=y.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>


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Hi, 

c/s 20194:582970a2d2dc overloads the system.

[BEFORE]
$ time xm create hvm.cfg memory=1024
Using config file "./hvm.cfg".
Started domain hvm (id=3)

real	0m2.343s
user	0m0.040s
sys	0m0.030s

[AFTER]
$ time xm create hvm.cfg memory=1024
Using config file "./hvm.cfg".
Started domain hvm (id=2)

real	2m25.741s
user	0m0.090s
sys	0m0.020s

I''m about to lose my patience with it. :-)
Can you control it by a boot parameter? (ex. "pod_debug=nnn")

Thanks,
Kouya

George Dunlap writes:
> The PoD code assumes that if:
> * A page is in a domain''s p2m table
> * And it''s owned by the domain
> * And it''s not a xenheap page
> then:
> * It''s on the domain''s page list.
> 
> This patch adds a check for this assumption when debug=y.
> 
> Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
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On 17/09/2009 02:26, "Kouya Shimura" <kouya@jp.fujitsu.com>
wrote:
> real 0m2.343s
> user 0m0.040s
> sys 0m0.030s
> 
> real 2m25.741s
> user 0m0.090s
> sys 0m0.020s
> 
> I''m about to lose my patience with it. :-)
> Can you control it by a boot parameter? (ex. "pod_debug=nnn")
Yeah, that''s too slow for our normal debug build. I think
we''ll revert the
patch -- noone would ever enable it, unless maybe if helping George track
down a PoD bug.

 -- Keir



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Keir Fraser wrote:
> On 17/09/2009 02:26, "Kouya Shimura" <kouya@jp.fujitsu.com>
wrote:
>
>   
>> real 0m2.343s
>> user 0m0.040s
>> sys 0m0.030s
>>
>> real 2m25.741s
>> user 0m0.090s
>> sys 0m0.020s
>>
>> I''m about to lose my patience with it. :-)
>> Can you control it by a boot parameter? (ex. "pod_debug=nnn")
>>     
>
> Yeah, that''s too slow for our normal debug build. I think
we''ll revert the
> patch -- noone would ever enable it, unless maybe if helping George track
> down a PoD bug.
>   
Hmm... my only concern is that the assumption seems like something which 
might easily change, with no one realizing that they''d broken the 
assumption, and the codepath not being tested since most OSS users 
probably won''t be using PoD.

Hmm, perhaps we could wrap it with an if() statement checking if there 
are any outstanding PoD entries?  The assumption only really matters 
when PoD is in use.

Alternately, a separate #define for PoD debug might be useful, so we can 
easily turn on all these options when we need to.

 -George

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