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SubjectRe: Can context switches be faster?
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Andrew James Wade wrote:
> On Friday 13 October 2006 01:29, John Richard Moser wrote:
>> True. You can trick the MMU into faulting into the kernel (PaX does
>> this to apply non-executable pages-- pages, not halves of VM-- on x86),
> Oooh, that is a neat hack!

Yes, very neat ;)

The way it works is pretty much that he has NX pages marked SUPERVISOR
so the userspace code can't put them in the TLB. What happens on access is:

- MMU cries, faults into kernel
- Kernel examines nature of fault
- If attempting to read/write, put the mapping in the DTLB for the
- If attempting to execute, KILL.
- Application continues running if it was a data access.

As I understand it, this is about 7000 times slower than having a
hardware NX bit and an MMU that handles the event; but once the page is
in the DTLB the MMU doesn't complain any more until it's pushed out, so
the hit is minimal. Wide memory access patterns that can push data out
of the TLB quickly and come back to it just as fast get a pretty
noticeable performance hit; but they're rare.

These days he's got the OpenBSD W^X/RedHat Exec Shield method in there
as well, so when possible the stack is made non-executable using
hardware and is completely exempt from these performance considerations.

- From what I hear, Linus isn't interested in either PTE hacks or segment
limit hacks, so old x86 will never have an NX bit (full like PaX or best
effort like Exec Shield) in the kernel. Too bad; I'd really like to see
some enforcement of non-executable pages on good old x86 chips in mainline.

>> but it's orders of magnitude slower as I understand and the petty gains
>> you can get over the hardware MMU doing it are not going to outweigh it.
> It's architecture-dependent; not all architectures are even capeable of
> walking the page table trees in hardware. They compensate with
> lightweight traps for TLB cache misses.

The ones that can do it probably have hardware tuned enough that a
software implementation isn't going to outrun them, least of all not far
enough to overtake the weight of the traps that can be set up. It's a
lovely area of theoretical hacks though, if you like coming up with
impractical "what kinds of nasty things can we do to the hardware"
ideas. :)

> Andrew Wade

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