2009237 - Disable spectre mitigations by default

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect, P3)

Description

[Jeff Muizelaar [:jrmuizel]]

This makes it so the shell has spectre mitigations off by default which
is less suprising given it's our default configuration on desktop.

It also fixes a problem where the behaviour didn't match the help text.
i.e. help says "Whether Spectre mitigations are enabled (default: off, on to enable)"
but spectre mitigations are actually on by default.

XPCJSContext.cpp sets these manually so browser builds are not affected.

Comment 1

[Jeff Muizelaar [:jrmuizel]]

Attachment: (secure)

This makes it so the shell has spectre mitigations off by default which
is less suprising given it's our default configuration on desktop.

It also fixes a problem where the behaviour didn't match the help text.
i.e. help says "Whether Spectre mitigations are enabled (default: off, on to enable)"
but spectre mitigations are actually on by default.

XPCJSContext.cpp sets these manually so browser builds are not affected.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/0a5ea8b6223c
https://hg.mozilla.org/integration/autoland/rev/f6e33f6175ea
Disable spectre mitigations by default r=rhunt,jandem

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/6aba9bef42f3
https://hg.mozilla.org/integration/autoland/rev/4638e157ee0d
Revert "Bug 2009237 - Disable spectre mitigations by default r=rhunt,jandem" for causing SM bustages.

Comment 4

[Atila Butkovits]

Backed out for causing SM bustages.

Backout link: https://hg-edge.mozilla.org/integration/autoland/rev/4638e157ee0dccc0d5598915ba697b4f2932428e

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&revision=f6e33f6175ea6dcee258b6e82c8f9688c9c2d45a&selectedTaskRun=Teri9B70SySkVRn49gmsFA.0

Failure log: https://treeherder.mozilla.org/logviewer?job_id=543248753&repo=autoland&task=Teri9B70SySkVRn49gmsFA.0&lineNumber=19247

Comment 5

[Iain Ireland [:iain]]

Is it safe to land this now?

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/730da4759675
https://hg.mozilla.org/integration/autoland/rev/b07032c6abae
Disable spectre mitigations by default r=rhunt,jandem

Comment 7

[Ryan VanderMeulen [:RyanVM]]

https://hg-edge.mozilla.org/mozilla-central/rev/b07032c6abae

Comment 8

[Mayank Bansal]

smattering of improvements on shell-only Js benchmarks on Linux

Comment 9

[Andra Esanu (needinfo me)]

(In reply to Pulsebot from comment #6)

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/730da4759675
https://hg.mozilla.org/integration/autoland/rev/b07032c6abae
Disable spectre mitigations by default r=rhunt,jandem

Perfherder has detected a js-bench performance change from push b07032c6abaed15e440945fc7b8d8cc46f1286ca.

No action is required from the author; this comment is provided for informational purposes only.

Improvement	Test	Platform	Options	Absolute values [old vs new]
5%	ares6-sm	linux64-shippable		41.93 ms -> 39.91 ms
5%	web-tooling-benchmark-sm	linux64-shippable		8.73 score -> 9.13 score

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Details of the alert can be found in the alert summary, including links to graphs and comparisons for each of the affected tests.