[webkit-dev] Disabling the 32-bit JITs by default.

[Guillaume Emont]

Quoting Filip Pizlo (2018-02-19 13:05:27)
> 
> > On Feb 19, 2018, at 10:53 AM, Guillaume Emont <guijemont at igalia.com> wrote:
> > 
> > Hi Keith,
> > 
> > We at Igalia have been trying to provide a better story for 32-bit
> > platforms, in particular for Armv7 and MIPS. These platforms are very
> > important to us, and disabling JIT renders many use cases impossible.
> 
> What use cases?

I'm not sure of how much I can elaborate here, but in this particular
case that was for a set-top-box UI.

> 
> I realize that having a JIT is good for marketing, but it’s better to have a stable and well-maintained interpreter than a decrepit JIT.  Right now the 32-bit JIT is basically unmaintained.

Indeed these platforms used to be practically abandoned in WebKit. I
don't think that is true any more though. We've been working on fixing
this and getting mips32 and armv7+thumb2 to pass all the tests. We have
achieved that for mips32[1] and we are almost there for armv7[2]. I
would appreciate it if you could acknowledge that effort.

[1] https://build.webkit.org/builders/JSCOnly%20Linux%20MIPS32el%20Release
[2] https://build.webkit.org/builders/JSCOnly%20Linux%20ARMv7%20Thumb2%20Release

> 
> > We
> > want to continue this effort to support these platforms. We have been
> > short on resources for that effort, which is why we did not realize
> > early enough that more mitigation was needed for 32-bit platforms. We
> > now have grown our team dedicated to this and we are hopeful that we
> > will avoid that kind of issue in the future.
> 
> I feel like I’ve heard this exact story before.  Every time we say that there isn’t any effort going into 32-bit, y’all say that you’ll put more effort into it Real Soon Now.  And then nothing happens, and we have the same conversation in 6 months.

I'm sorry it took us time to grow our team for this purpose, but that is
now a reality since the beginning of this month. And beside that, I
think you can agree that there has been significant progress on that
aspect, we were very far from having a green tree on mips32 about a year
ago, when we still had hundreds of test failures.

> 
> > 
> > We are working on a plan to mitigate Spectre on 32-bit platforms. We
> > would welcome community feedback on that, as well as what kinds of
> > mitigations would be considered sufficient.
> > 
> > Regarding your patch, I think you should note that some specific 32-bit
> > CPUs are immune to Spectre (at least the Raspberry Pi[1] and some
> > MIPS[2] devices), I think the deactivation should be done at run-time
> > for CPUs not on a white list.
> 
> Keith’s main point is that the presence of 32-bit makes it harder to implement mitigations for 64-bit.  I don’t think it’s justifiable to hold back development of 64-bit Spectre mitigations because of a hardly-used and mostly-broken 32-bit JIT port that will be maintained by someone Real Soon Now.

I can't answer to that as I don't know enough what is hindering these
mitigations exactly.


Best regards,

Guillaume