On Wed, Sep 18, 2002 at 10:38:38AM +0200, Kevin D. Kissell wrote: > From: "Jun Sun" <jsun@mvista.com> > > On Wed, Sep 18, 2002 at 01:44:57AM +0200, Kevin D. Kissell wrote: > > > > > > 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. > In all places the task is "current" process. Therefore no inter-processor traffic. Obiviously it is still less desriable than a bit in cpu regiters.... Jun
