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> At what point does the complexity of CPU architectures become so difficult to reason about that we just accept the performance penalty of keeping it simpler?

> At what point does the complexity of CPU architectures become so difficult to reason about that we just accept the performance penalty of keeping it simpler? Never for branch prediction. It just gets you too much performance. If it becomes too much of a problem, the solution is greater isolation of workloads.

hedgehog 3y ago
In certain cases isolation and simplicity overlap, I suspect for example that the dangers of SMT implementation complexity are part of why Apple didn't implement it for their respective CPUs. Likely we'll see this elsewhere too, for example Amazon may not ever push to have SMT in their Graviton chips (the early generations are off the shelf cores from ARM where they didn't have a readily available choice).
- asdfasgasdgasdg 3y ago
  I could be mistaken, but I don't think Zenbleed has anything to do with SMT, based on my reading of the document. There is a mention of hyperthreads sharing the same physical registers, but you can spy on anything happening on the same physical core, because the register file is shared across the whole core. It even says so in the document: Note that it is not sufficient to disable SMT. Apple's chips don't have this vulnerability, but it's not because they don't have SMT. They just didn't write this particular defect into their CPU implementation.
  - hedgehog 3y ago
    Correct, I was responding to parent writing "At what point does the complexity of CPU architectures become so difficult to reason about that we just accept the performance penalty of keeping it simpler?" I think we may be seeing an industry-wide shift away from SMT because the performance penalty is small and the complexity cost is high, if so that fits parent's speculation about the trend. In a narrow sense Zenbleed isn't related to SMT but OP's question seems perfectly relevant to me. I come from a security background and on average more complicated == less secure because engineering resources are finite and it's just harder and more work to make complicated things correct.
- meithecatte 3y ago
  Eh, as long as you assign both hyper-threads to the same tenant, and schedule them at the same time, you should be fine.
  - throwawaylinux 3y ago
    Not really if that's an attack you're concerned about, because guests can attack the hypervisor via the same mechanisms. You would need to gang schedule to ensure all threads of a core were only either in host or guest.

hedgehog 3y ago

In certain cases isolation and simplicity overlap, I suspect for example that the dangers of SMT implementation complexity are part of why Apple didn't implement it for their respective CPUs. Likely we'll see this elsewhere too, for example Amazon may not ever push to have SMT in their Graviton chips (the early generations are off the shelf cores from ARM where they didn't have a readily available choice).

asdfasgasdgasdg 3y ago
I could be mistaken, but I don't think Zenbleed has anything to do with SMT, based on my reading of the document. There is a mention of hyperthreads sharing the same physical registers, but you can spy on anything happening on the same physical core, because the register file is shared across the whole core. It even says so in the document: Note that it is not sufficient to disable SMT. Apple's chips don't have this vulnerability, but it's not because they don't have SMT. They just didn't write this particular defect into their CPU implementation.
- hedgehog 3y ago
  Correct, I was responding to parent writing "At what point does the complexity of CPU architectures become so difficult to reason about that we just accept the performance penalty of keeping it simpler?" I think we may be seeing an industry-wide shift away from SMT because the performance penalty is small and the complexity cost is high, if so that fits parent's speculation about the trend. In a narrow sense Zenbleed isn't related to SMT but OP's question seems perfectly relevant to me. I come from a security background and on average more complicated == less secure because engineering resources are finite and it's just harder and more work to make complicated things correct.
meithecatte 3y ago
Eh, as long as you assign both hyper-threads to the same tenant, and schedule them at the same time, you should be fine.
- throwawaylinux 3y ago
  Not really if that's an attack you're concerned about, because guests can attack the hypervisor via the same mechanisms. You would need to gang schedule to ensure all threads of a core were only either in host or guest.