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Re: allowed uses of non-baseline CPU extensions

On Thu, Oct 05, 2017 at 03:52:56AM +0200, Adam Borowski wrote:
> But, Adrian Bunk warned that this makes violating the baseline too easy.
> And indeed, I just noticed an attempt to use an extension in a way I don't
> consider to be valid: #864012.  I understand the maintainer's reasoning,
> and don't blame him for following recommendations of that package's
> upstream, but it's not appropriate for what I assume our _unwritten_ rules
> mean.  And here's the problem: I can't seem to find an explicit requirement
> that packages must follow the baseline!  So let's discuss and make a policy.

The policy so far has been "baseline violations are RC bugs".

And while your intentions are laudable, you are solving a small problem 
but creating a huge problem.

For pcsx2, an obscure emulator that only works on i386 and requires SSE2,
I am saying meh and see how a dependency on sse2-support is actually an

Note that the number of such packages is very low, usually there is
portable code with the problematic code (build time or runtime) optional.

The problem is that blessing baseline violations with a dependy on 
*-support is completely screwing the baseline.

One interesting aspect of baseline violations are upgrades
to a new stable.

The documented way to upgrade jessie -> stretch is:
apt-get upgrade
apt-get dist-upgrade

Think of at what point newly added *-support dependencies 
will become visible during stretch -> buster upgrades.

And we are not only screwing our users, we are even scewing ourselves:

What happens if a build dependency pulls in neon-support?
None of our current armhf buildds supports NEON.

And as you already noticed in #864012, sse4.2-support has the 
same problem on some amd64 buildds.

For a real-life example, let's look at #871700.
There are three things that are worth noticing:

First, we do have users running unstable on an original Pentium III.

Second, the only reason why Firefox still works there is that the
maintainer defended the baseline by not using SSE2 as upstream does.

Third, the bug was reported against nss.
Following upstream and adding an sse2-support dependency to libnss3
would not only remove Firefox, it would also imply no LibreOffice 
and no LaTeX without SSE2.

Next we can discuss the severity of #877445.
Is this bug RC, or will we add an sse2-support depencency in a DSA for 
Firefox in stretch next summer - leaving people at the i386 baseline 
with no supported browser?

Another interesting question is which Go compiler to use on s390x:
- golang-go builds all Go packages but does not support the baseline
- gccgo does support the baseline but cannot build all packages

As last example, let's look at QtWebEngine:

Chromium is one of these few application using SSE2 on i386 without any 
portable code version that could be used instead.

QtWebEngine (src:qtwebengine-opensource-src) is based on Chromium
and is starting to be heavily used by Qt code (including KDE).

QtWebEngine is not available on all architectures, so code using it 
already has to either reduce functionality or use the WebKit-based 
QtWebKit (src:qtwebkit-opensource-src) on other architectures.

There are two options availabe:
1. RC bug that qtwebengine-opensource-src must not be built on i386
   since it violates the baseline, all code using it has to do whatever
   mitigation is already done for the other architectures without
2. bless the SSE2 usage with a dependency on sse2-support, making
   KDE and several other Qt applications not available without SSE2



       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed