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the RNAT bit index constant. The net effect of this is that there's no discontinuity WRT NaT collections which greatly simplifies certain operations. The cost of this is that there can be up to 504 bytes of unused stack between the true base of the kernel stack and the start of the RSE backing store. The cost of adjusting the backing store pointer to keep the RNAT bit index constant, for each kernel entry, is negligible. The primary reasons for this change are: 1. Asynchronuous contexts in KSE processes have the disadvantage of having to copy the dirty registers from the kernel stack onto the user stack. The implementation we had so far copied the registers one at a time without calculating NaT collection values. A process that used speculation would not work. Now that the RNAT bit index is constant, we can block-copy the registers from the kernel stack to the user stack without having to worry about NaT collections. They will be in the right place on the user stack. 2. The ndirty field in the trapframe is now also usable in userland. This was previously not the case because ndirty also includes the space occupied by NaT collections. The value could be off by 8, depending on the discontinuity. Now that the RNAT bit index is contants, we have exactly the same number of NaT collection points on the kernel stack as we would have had on the user stack if we didn't switch backing stores. 3. Debuggers and other applications that use ptrace(2) can now copy the dirty registers from the kernel stack (using ptrace(2)) and copy them whereever they want them (onto the user stack of the inferior as might be the case for gdb) without having to worry about NaT collections in the same way the kernel doesn't have to worry about them. There's a second order effect caused by the randomization of the base of the backing store, for it depends on the number of dirty registers the processor happened to have at the time of entry into the kernel. The second order effect is that the RSE will have a better cache utilization as compared to having the backing store always aligned at page boundaries. This has not been measured and may be in practice only minimally beneficial, if at all measurable.
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