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Seems like speculative execution is just fundamentally insecure. With SPECTRE/MELTDOWN mitigations, doesn't CPU performance drop below the same CPU performance

Seems like speculative execution is just fundamentally insecure. With SPECTRE/MELTDOWN mitigations, doesn't CPU performance drop below the same CPU performance with no branch prediction at all? Should we move back to CISC? Or maybe VLIW?

andrewia 2y ago
I don't think so; speculative execution is the cornerstone of modern CPU performance. Even 15-year-old 32-bit ARM CPUs do it. The only phone/PC-grade processors without it are the first generation of Intel Atom, and I recall that early Atom processors sacrificed a ton of performance to keep power consumption low. I doubt this will change since mitigations are "good enough" to patch over major issues.
- happosai 2y ago
  There is extremely popular Cortex-A53 which is in-order core.
  - MindSpunk 2y ago
    Yes and it's very slow as a result. In-order cores without speculative execution can't be fast. Not unless you have no memory and only operate out of something equivalent to L1 cache. Memory is slow. Insanely slow (compared to the CPU). You can process stupid fast if your entire working set can fit in a 2KB L1 cache, but the second you touch memory you're hosed. You can't hide memory latency without out-of-order execution and/or SMT. You fundamentally need to be parallel to hide latency. CPUs do it with out-of-order and speculative execution. GPUs do it by being stupidly parallel and running something like 32-64 way SMT (huge simplification). Many high-performance CPUs do all of these things. Instruction level parallelism is simply not optional with the DRAM latency we have.
    - happosai 2y ago
      Cortex a53 may be slow, but it's fast enough for very many tasks. Once you design the your data structures to fit L1/L2 caches it actually is pretty damn fast. Best part of cache aware data structure design it also makes code run faster on Out Of Order CPUS. A53 is of course slow if you use modern layer-upon-layer-ware as your architecture. But I was just really trying to point that in-order cpus are still around, they did not disappear with in-order atom.
- dailykoder 2y ago
  Maybe the boomers were right and we made computers way too complex? This might be a bit of a hyperbole, but seems like there will always be a security hole (even if mostly hard to exploit). But I also guess we can't get much faster without it either. So maybe we should reduce complexity. Atleast for safety critical systems.
  - MagicMoonlight 2y ago
    Now wait until the zoomers come along and take the lead on these products. They grew up with iPads and no file system. It’s going to be chaos!
    - saagarjha 2y ago
      Boomers grew up without a filesystem too and things seem to have worked out fine.
School-Cotton 2y ago
> SPECTRE/MELTDOWN mitigations, doesn't CPU performance drop below the same CPU performance with no branch prediction at all? No. Processors with no branch prediction would be many times slower (up to dozens of times slower on workloads that don’t fit in cache)
rasz 2y ago
>speculative execution is just fundamentally insecure I dont think its inevitable, might be caused by greed. You could have small separate sections of cache (or additional tag) dedicated to per thread speculation, but no designer is willing to sacrifice real estate to something that will be thrown away the instantly.

andrewia 2y ago

I don't think so; speculative execution is the cornerstone of modern CPU performance. Even 15-year-old 32-bit ARM CPUs do it. The only phone/PC-grade processors without it are the first generation of Intel Atom, and I recall that early Atom processors sacrificed a ton of performance to keep power consumption low. I doubt this will change since mitigations are "good enough" to patch over major issues.

happosai 2y ago
There is extremely popular Cortex-A53 which is in-order core.
- MindSpunk 2y ago
  Yes and it's very slow as a result. In-order cores without speculative execution can't be fast. Not unless you have no memory and only operate out of something equivalent to L1 cache. Memory is slow. Insanely slow (compared to the CPU). You can process stupid fast if your entire working set can fit in a 2KB L1 cache, but the second you touch memory you're hosed. You can't hide memory latency without out-of-order execution and/or SMT. You fundamentally need to be parallel to hide latency. CPUs do it with out-of-order and speculative execution. GPUs do it by being stupidly parallel and running something like 32-64 way SMT (huge simplification). Many high-performance CPUs do all of these things. Instruction level parallelism is simply not optional with the DRAM latency we have.
  - happosai 2y ago
    Cortex a53 may be slow, but it's fast enough for very many tasks. Once you design the your data structures to fit L1/L2 caches it actually is pretty damn fast. Best part of cache aware data structure design it also makes code run faster on Out Of Order CPUS. A53 is of course slow if you use modern layer-upon-layer-ware as your architecture. But I was just really trying to point that in-order cpus are still around, they did not disappear with in-order atom.
dailykoder 2y ago
Maybe the boomers were right and we made computers way too complex? This might be a bit of a hyperbole, but seems like there will always be a security hole (even if mostly hard to exploit). But I also guess we can't get much faster without it either. So maybe we should reduce complexity. Atleast for safety critical systems.
- MagicMoonlight 2y ago
  Now wait until the zoomers come along and take the lead on these products. They grew up with iPads and no file system. It’s going to be chaos!
  - saagarjha 2y ago
    Boomers grew up without a filesystem too and things seem to have worked out fine.

School-Cotton 2y ago

> SPECTRE/MELTDOWN mitigations, doesn't CPU performance drop below the same CPU performance with no branch prediction at all? No. Processors with no branch prediction would be many times slower (up to dozens of times slower on workloads that don’t fit in cache)

rasz 2y ago

>speculative execution is just fundamentally insecure I dont think its inevitable, might be caused by greed. You could have small separate sections of cache (or additional tag) dedicated to per thread speculation, but no designer is willing to sacrifice real estate to something that will be thrown away the instantly.