> > True, though it should perhaps be noted that currently it's only on 4KSc/Sd
> > systems (which I know you work on) where it's even possible for the stack
> > *not* to have exec permissions, since the classical MIPS MMU gives
> > execute permission to any page that is readable.
>
> Disabling PROT_EXEC on a mapping tells the kernel it doesn't need to take
> care of I-cache coherency. So it's somewhat beneficial even in absence of
> a protection bit in the actual TLB hardware.
That depends on just what the consequences of I-cache incoherence might be.
Without help from the MMU, the kernel cannot *know* that a given location
isn't in the I-cache, because a program can always compute a pointer-to-function
to an arbitrary address and dereference it successfully so long as the location
is readable. If it's only the user who does this that will suffer as a result of
I-cache incoherence, one can argue that it serves him right. But if it can screw
up the execution of the kernel, or other user processes, I think we have to
assume the worst.
> Some of these performance optimizations are impossible because the kernel
> can't have definate knowledge that certain addresses have never entered the
> I-cache.
Sad but true.
Regards,
Kevin K.
