On Tue, Oct 19, 2010 at 11:14 PM, Ali Saidi <[email protected]> wrote:
>
> I think the prefetch should be sent the the TLB unconditionally, and then if
> the prefetch faults the CPU should toss the instruction rather than the TLB
> returning no fault and the CPU i guess checking if the PA is set?
>
> I agree that we should override the fault in the CPU. Are we violently
> agreeing?
OK, it's becoming a little clearer to me now. I think we're agreeing
that the TLB should be oblivious to whether an access is a prefetch or
not, so that's a start.
The general picture I'd like to see is that once a prefetch returns
from the TLB, the CPU does something like:
if (inst->fault == NoFault) {
access the cache
} else if (inst->isPrefetch()) {
maybe set a flag if necessary
inst->fault = NoFault;
}
...so basically everywhere else down the pipeline where we check for
faults we don't have to explicitly except prefetches from normal fault
handling.
If there are points past this one where we really care to know if a
prefetch accessed the cache or not, then maybe we need a flag to
remember that (sort of a dynamic version of the NO_ACCESS static
flag), but I don't know if that's really necessary or not. Clearly if
the cache access doesn't happen right there, then we can add the flag
and use it later to decide whether to access the cache.
Anyway, this is the flavor I was going for... any issues with it?
Steve
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