Or use a PLB

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), September 27, 2021 11:58 am
Room: Moderated Discussions
Linus Torvalds (torvalds.delete@this.linux-foundation.org) on September 27, 2021 10:20 am wrote:
> Oh, the implementation differences can be huge. Do you "integrate" the pointer
> with the capability data like Cheri does? Or do you keep them independent so
> that you can use one "segment" with multiple indexes (like i386 did)?

Thinking about it, the biggest difference is not that the i386 model made the segment be very clearly separate from the pointer, but that the i386 segment model itself was based on indirection and explicit software management of segments.

IOW, you never loaded "a segment". You loaded a "segment selector", and the hardware then looked up the actual segment data from a "segment table" (actually, two different ones, but at that point we're going off into "irrelevant implementation details" weeds).

That makes perfect sense in one way, but one of the limitations in that segment model was that it didn't allow users of segments to easily "sub-segment" things or do other things on their own. The only thing you as a user saw was the selector value, and while you could query certain state (like the segment limit), you couldn't really do operations on segments - you had to create a new entry in the segment table.

Contrast this to the Cheri model, which doesn't have that indirection through a segment table, and you can create a new segment ("capability pointer") from an existing valid one. So you can take a big segment, and create a new smaller subset of it (or a read-only one from a read-write one) and just use it without going through that "segment table" thing.

Is that "fundamentally different"? No. The i386 model is one of an explicit software-managed segment table, while the Cheri one is comes with hardware support for managing your segments.

Does the Cheri model seem much better? Ahh, there's the nub. It sure as heck looks a lot cleaner than i386 segments did (I say "i386", but it was really the 286 that introduced them).

But looks can be deceiving.

The Cheri model requires hidden bits of memory from hardware, for example. The Cheri model also makes all pointers really really big.

So it's not some kind of "Cheri is obviously better". Cheri looks much nicer, but it has serious downsides, and the big difference is that we know the i386 model sucked, we don't have the same kind of lots of experience with the Cheri model.

With Cheri you can do pointer compression (and if Cheri were to become popular, I bet people would try to do so), and you'd probably end up making your data structures use a lot of "index off a base pointer" instead of using a pointer. Because a 128-bit pointer is just really really big and wastes a lot of memory (it also has to be 128-bit aligned, because of how that verification bit works).

That "pointer compression" was actually very very natural in the i386 model. It is not natural in the Cheri model. It requires that "one single base pointer". And that's fundamentally what the "segment selector" is.

And yes, we've seen this before: people did some of that kind of work when moving to 64-bit architectures, and it sure wasn't all fun and games. On the whole, most projects kept using pointers, rather than "index plus base pointer", because managing the base pointer is just too painful, and it really doesn't work well when you have multiple allocations.

And that "too painful" becomes even more true when you have segments / capability pointers and multiple allocations.

Maybe the i386 segment model with pointer compression using explicit near and far pointers wasn't wrong after all?

See? i386 segments really aren't so conceptually different from Cheri capability pointers. Very different implementation choices, and very different trade-offs. We know the i386 segment model was horrible to use. Does Cheri fix the problems? It's not immediately and trivially obvious. That "use index off a base pointer" doesn't look so different from near and far pointers, now does it?

There's a reason Cheri is a "research project".

But there is no real reason to claim that capabilities are fundamentally different from segments.

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TopicPosted ByDate
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                                          Or use a PLB---2021/09/16 12:02 PM
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                Virtually tagged L1-cachessr2021/09/11 12:19 AM
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                  Virtually tagged L1-caches---2021/09/11 08:53 AM
                    Virtually tagged L1-cachessr2021/09/11 11:43 PM
                      Virtually tagged L1-cachesLinus Torvalds2021/09/12 10:10 AM
                        Virtually tagged L1-cachessr2021/09/12 10:57 AM
                          Virtually tagged L1-cachesdmcq2021/09/13 07:31 AM
                            Virtually tagged L1-cachessr2021/09/20 03:11 AM
            Virtually tagged L1-cachessr2021/09/11 01:49 AM
      Virtually tagged L1-cachesLinus Torvalds2021/09/08 11:34 AM
        Virtually tagged L1-cachesdmcq2021/09/09 01:46 AM
          Virtually tagged L1-cachesdmcq2021/09/09 01:58 AM
          Virtually tagged L1-cachessr2021/09/11 12:29 AM
            Virtually tagged L1-cachesdmcq2021/09/11 07:59 AM
              Virtually tagged L1-cachessr2021/09/11 11:57 PM
                Virtually tagged L1-cachesdmcq2021/09/12 07:44 AM
                  Virtually tagged L1-cachessr2021/09/12 08:48 AM
                    Virtually tagged L1-cachesdmcq2021/09/12 12:22 PM
                      Virtually tagged L1-cachessr2021/09/20 03:40 AM
    Where do you see this information? (NT)anon22021/09/09 01:45 AM
      Where do you see this information?sr2021/09/11 12:40 AM
        Where do you see this information?anon22021/09/11 12:53 AM
          Where do you see this information?sr2021/09/11 01:08 AM
            Thank you (NT)anon22021/09/11 03:31 PM
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