From: "Jun Sun" <redacted>

On Wed, Sep 18, 2002 at 01:44:57AM +0200, Kevin D. Kissell wrote:

quoted

I'd much prefer something that is simple and processor-local,
even if it may be less optimal in some corner cases.  For example,
Why not simply use CP0.Status.CU1 as a "dirty" bit?  If it's set 
when a process switches out, the FPU state gets saved, and CU1 
cleared.  If it's not set when a process hits an FP instruction, 
CU1 gets set and the context gets loaded. This involves no 
access whatever to shared control variables, indeed, it doesn't 
even go to memory to make the decision. It will, of course, save 
some FP contexts that don't need saving, but it is well behaved
in the cases I care most about - it avoids saving/restoring FPRs
of code that is doing no FP whatsoever, and it ensures that
whenever a thread starts up, whatever CPU its on, its full
context is available to that CPU, no (coherent) questions asked.

This is basically 2) except for dirty bit difference.

My current implementaion uses bit:1 in task->used_math flag for 
"dirty" bit purpose.

Which is not a property of the CPU, but of the thread,
meaning that it will be written by one CPU and read by
another, i.e. there will be MP memory traffic and cache
interventions/invalidations/misses around the operation.

            Kevin K.