[mkc2008] Platform-Specific Optimizations - Small Address Spaces (IA-32)

Torsten Frenzel frenzel at os.inf.tu-dresden.de
Mon Apr 6 18:16:25 CEST 2009


Hi,

Jan Bierbaum(s3306700 at mail.inf.tu-dresden.de)@2009.04.06 16:51:05 +0200:
> Hello!
> 
> 
> On slides 12-14 Small Address Spaces are introduced: To save the
> expenses of address space switching, some small AS are established
> inside the kernel memory region. Mutual protection and relocation is
> provided through segmentation.
> 
> But since this memory is located inside the kernel region wouldn't that
> mean the tasks inside those small AS need kernel privileges to access
> this memory? I cannot believe this, because it would also allow them to
> execute privileged instructions.
> 
Just that a user-level program has access to a region above 3GB doesn't
mean that it needs kernel privileges and has access to kernel memory.
The SMAS application has only access to a dedicated part in the kernel
region that is mapped user accessible. (Therefore it can't execute privileged
instructions because it runs with user privileges.)

> An alternative would be that with small AS there is no "kernel memory"
> at all - access to all pages is allowed with user privilege (in the page
> tables) and isolation is done solely via segmentation. So segmentation
> does not only isolate the small AS from each other but also the normal
> user AS from kernel memory. Is that was is done?
>
In principle yes. All SMAS applications and the kernel live in the same
address space (using the same page table) and are isolation via segmentation.

Torsten



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