Stream: git-wasmtime

Topic: wasmtime / PR #12990 MMU-based epoch interruption

Wasmtime GitHub notifications bot (Apr 08 2026 at 16:33):

erikrose opened PR #12990 from erikrose:epoch-mmu to bytecodealliance:main:

This is an implementation of #1749, specifically @cfallin's roadmap, with the goal of reducing the overhead of checking for the end of epochs.

Paul ran some benchmarks on this (broadly agreeing with our real-world experiments) which tell us:

• Current compare-against-a-deadline -Wepoch-interruption=y is a 14.4% hit versus doing nothing.
• Doing only dead loads in function prologues and loop headers (which were all that was implemented in this patch at the time of the bench) brings that down to a 2.8% hit. There will be some additional hit from the signal handler that actually effects the task switch, but that's on the cold path.

The above numbers are from SpiderMonkey, which I deem the most representative benchmark.

Status:

• [X] Add unmapped-on-interrupt page and pointer to it in vmctx.
• [X] Add method for embedder to call to bring an epoch to a close.
• [X] Add DeadLoadWithContext Cranelift instruction, and use it.
• [ ] Have DeadLoadWithContext emit metadata into compiled-artifact tables to tell the signal handler this is an interruption-point load.
• [ ] Add logic to signal handler that, when seeing such a PC, updates state to redirect to the stub, saving the original PC (probably in the scratch register).
• [ ] Add that stub, which saves all register state and invokes a hostcall with the recovered vmctx.

If the TLB shootdown arising from the frobbing of privs on the "interrupt page" proves too expensive, we can try a more indirect load instead, where, instead of messing with page privs, we mess with the address we're dead-loading from so it points to either a (statically) allowed or forbidden page. (Chris floated this idea at the 2026-04-08 Cranelift meeting.) Not many of the other mechanics need change.

Wasmtime GitHub notifications bot (Apr 08 2026 at 20:00):

github-actions[bot] added the label cranelift on PR #12990.

Wasmtime GitHub notifications bot (Apr 08 2026 at 20:00):

github-actions[bot] added the label cranelift:area:x64 on PR #12990.

Wasmtime GitHub notifications bot (Apr 08 2026 at 20:00):

github-actions[bot] added the label cranelift:area:machinst on PR #12990.

Wasmtime GitHub notifications bot (Apr 08 2026 at 20:00):

github-actions[bot] added the label cranelift:docs on PR #12990.

Wasmtime GitHub notifications bot (Apr 08 2026 at 20:00):

github-actions[bot] added the label wasmtime:config on PR #12990.

Wasmtime GitHub notifications bot (Apr 08 2026 at 20:00):

github-actions[bot] added the label wasmtime:api on PR #12990.

Wasmtime GitHub notifications bot (Apr 08 2026 at 20:00):

github-actions[bot] added the label cranelift:meta on PR #12990.

Wasmtime GitHub notifications bot (Apr 08 2026 at 20:02):

github-actions[bot] commented on PR #12990:

Label Messager: wasmtime:config

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[fuzzing-docs]: https://docs.wasmtime.dev/contributing-fuzzing.html

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Wasmtime GitHub notifications bot (Apr 10 2026 at 21:50):

erikrose updated PR #12990.

Wasmtime GitHub notifications bot (Apr 10 2026 at 21:50):

erikrose edited PR #12990:

This is an implementation of #1749, specifically @cfallin's roadmap, with the goal of reducing the overhead of checking for the end of epochs.

Paul ran some benchmarks on this (broadly agreeing with our real-world experiments) which tell us:

• Current compare-against-a-deadline -Wepoch-interruption=y is a 14.4% hit versus doing nothing.
• Doing only dead loads in function prologues and loop headers (which were all that was implemented in this patch at the time of the bench) brings that down to a 2.8% hit. There will be some additional hit from the signal handler that actually effects the task switch, but that's on the cold path.

The above numbers are from SpiderMonkey, which I deem the most representative benchmark.

Status:

• [X] Add unmapped-on-interrupt page and pointer to it in vmctx.
• [X] Add method for embedder to call to bring an epoch to a close.
• [X] Add DeadLoadWithContext Cranelift instruction, and use it.
• [x] Have DeadLoadWithContext emit metadata into compiled-artifact tables to tell the signal handler this is an interruption-point load.
• [ ] Add logic to signal handler that, when seeing such a PC, updates state to redirect to the stub, saving the original PC (probably in the scratch register).
• [ ] Add that stub, which saves all register state and invokes a hostcall with the recovered vmctx.

If the TLB shootdown arising from the frobbing of privs on the "interrupt page" proves too expensive, we can try a more indirect load instead, where, instead of messing with page privs, we mess with the address we're dead-loading from so it points to either a (statically) allowed or forbidden page. (Chris floated this idea at the 2026-04-08 Cranelift meeting.) Not many of the other mechanics need change.

Last updated: Apr 13 2026 at 00:25 UTC