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

Jan Bierbaum s3306700 at mail.inf.tu-dresden.de
Mon Apr 6 16:51:05 CEST 2009


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.

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?

Thanks, Jan.

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