Re: [XenAccess-devel] PSE check

[Bryan D. P. <br...@th...>]

The latest version of the code is currently undergoing a lot of  
changes with the ultimate goal of supporting windows.  The PSE check  
was added (along with other checks) to make sure that the page table  
layout is supported by xenaccess before trying to walk the page table  
and, possibly, interpreting the entries incorrectly.

PSE is an interesting beast.  Just because the bit is enabled in CR4  
doesn't mean that variable sized pages are actually in use.  It just  
means that they might be in use.  Right now the code fails on the PSE  
check because my page table lookup code does not check the PTE bit to  
see what size the pages are.  It probably works just fine for you  
because you are working on a system (or at least a part of a system)  
that has 4k pages.

In general, there is no harm in just commenting out this code for  
now, if you'd like.  This will provide the same functionality as  
previously was available with xenaccess.  The only real risk is that  
the system may not work properly if you have a 4MB page.  Before the  
next release, I plan on actually implementing the proper page table  
lookup techniques so that this whole issue goes away (i.e., I'll  
perform the proper checks so that the failure condition that you are  
currently seeing will not exist).

 From what I have seen, PV domains do not have the PSE bit set  
whereas HVM domains do.  So if you're working with HVM, it would make  
sense that this check is giving you trouble.

Sorry for the long answer, but hopefully that makes sense.

Cheers,
bryan


On Aug 15, 2007, at 3:26 PM, Hajime Inoue wrote:

> A recent update to subversion added a check for PSE.
> This is enabled in my images.  However, the examples
> work when this check is commented out.  Is this
> a paravirtualized-only problem, or one where big pages
> could cause problems, but since I'm not using big pages,
> I don't see one?
>
> Thanks,
> -Jim


-
Bryan D. Payne
Graduate Student, Computer Science
Georgia Tech Information Security Center
http://www.bryanpayne.org