[mkc2008] Platform-Specific Optimizations - Small Address Spaces (IA-32)
Udo A. Steinberg
udo at hypervisor.org
Mon Apr 6 18:22:26 CEST 2009
On Mon, 06 Apr 2009 16:51:05 +0200 Jan Bierbaum (JB) wrote:
JB> On slides 12-14 Small Address Spaces are introduced: To save the
JB> expenses of address space switching, some small AS are established
JB> inside the kernel memory region. Mutual protection and relocation is
JB> provided through segmentation.
JB> But since this memory is located inside the kernel region wouldn't that
JB> mean the tasks inside those small AS need kernel privileges to access
JB> this memory? I cannot believe this, because it would also allow them to
JB> execute privileged instructions.
JB> An alternative would be that with small AS there is no "kernel memory"
JB> at all - access to all pages is allowed with user privilege (in the page
JB> tables) and isolation is done solely via segmentation. So segmentation
JB> does not only isolate the small AS from each other but also the normal
JB> user AS from kernel memory. Is that was is done?
You can find all the details about the original implementation of small
spaces in the following paper from Jochen Liedtke:
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