RE: [11/14] vcompound: Fallbacks for order 1 stack allocations on IA64 and x86

[Luck, Tony]

> I thought the only pinned TLB entry was for the per cpu area? How does it 
> pin the TLB? The expectation is that a single TLB covers the complete 
> stack area? Is that a feature of fault handling?

Pinning TLB entries on ia64 is done using TR registers with the "itr"
instruction.  Currently we have the following pinned mappings:

itr[0] : maps kernel code.  64MB page at virtual 0xA000000100000000
dtr[1] : maps kernel data.  64MB page at virtual 0xA000000100000000

itr[1] : maps PAL code as required by architecture

dtr[1] : maps an area of region 7 that spans kernel stack
         page size is kernel granule size (default 16M).
         This mapping needs to be reset on a context switch
         where we move to a stack in a different granule.

We used to used dtr[2] to map the 64K per-cpu area at 0xFFFFFFFFFFFF0000
but Ken Chen found that performance was better to use a dynamically
inserted DTC entry from the Alt-TLB miss handler which allows this
entry in the TLB to be available for generic use (on most processor
models).

-Tony

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