1734202 - (Maybe) Move spectre mitigation flags from JitOptions to ContextOptions

Status: RESOLVED WONTFIX

(Core :: JavaScript Engine, enhancement, P3)

Description

[Tom Ritter [:tjr]]

It seems likely that we will want to enable Spectre Mitigations for file: origins but not other origins (Bug 1712580). Presently, because Spectre Mitigations are on JitOptions they are process-wide, which happens to work because all file: origins are in their own process but we may want the flexibility to enable (or disable) spectre mitigations on a more granular level, especially on Mobile.

This is a TBD work item, but getting it on file for now.

Comment 1

[Jan de Mooij [:jandem]]

ContextOptions are tied to the JSContext/thread. I think you'd want something more granular based on Realm or Compartment, but because realms/compartments in the same Zone can share JIT code that gets complicated... There are also trampolines that are shared per context/thread.

I'd really prefer to keep these per-process because anything more granular makes it hard to ensure we set them consistently for all different threads (main thread, workers, worklets, PAC thread, etc).

Comment 2

[Hsin-Yi Tsai (she/her) [:hsinyi]]

Per offline confirmation with Tom and Nika - we can WONTFIX this for now because we don't think we will need this use case.