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Re: About VMX/Altivec support in Debian ppc64 port

Hi, All.
I am a ppc64 porter, Hiroyuki Yamamoto.

Because ppc64 port is NOT a official port yet,
please DON'T report to official BTS the bugs which have NOT a relation to the packages AT ALL,
and which have a relation ONLY to porting to ppc64 port,

Instead, please report to this debian-powerpc ML, and for making sure,
please add ppc64@debian-ports.org (ppc64 buildd maintainers of Debian Ports) to CC.

Lennart Sorensen wrote:
On Sat, Feb 16, 2013 at 12:07:09AM +0800, Liang Guo wrote:
I'm working to run Debian ppc64 ports on my xbox360 box, but
failed to setup a chrooted environment with "Illegal instruction"
error. After some dig on this problem, I found this is caused
by PowerPC vector instrunctions, and according to the build
log[1], the package do compile with " -maltivec -mabi=altivec ".

On this time, unless the package maintainer is fixing build option specially, the packages of ppc64 port are built
with build options "-pipe -maltivec -mabi=altivec -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt".

This has an experimental intention for evaluating merits and demerits of building with these build options,
and being able to run various powerpc64 machines.

Because official powerpc port also support powerpc64 machines,
I think that machines supported by ppc64 port may be able to restricted to some extent.
Further build options which should be adopted may be adopted,
but I also think that it is not good to carry out FATAL restriction to ppc64.

So, I run ppc64 porting and offer the ppc64 packages, and I wait reports of bugs, benchmarks, opinions, and so on.
This report is the first one, so I welcome this report very much.

So I want know, why we use "-maltivec" in PPC64 ports, is
it faster then altivec disabled? After all, many old Power4/5,
and qemu ppc64 simulator don't support altivec.

I didn't consider for qemu.
It may be inconvenience that it cannot use as an image of qemu.

About not supporting for POWER4/5 machines, it is intentional on this time powerpc port already existing.

According Debian ppc64 wiki page[2], xenon, the cpu used by
xbox360, supports altivec, but altivec don't work in my box,
I think it is another bug or misconfigure.

I'm not a subscriber of this list, please CC me.

[1] http://buildd.debian-ports.org/status/fetch.php?pkg=util-linux&arch=ppc64&ver=2.20.1-5.3&stamp=1355261928
[2] http://wiki.debian.org/PPC64

Is it wrong although I think that Xenon CPUs support the VMX instructions?

However, I think that the phenomenon which ppc64 port doesn't work on Xbox360 with really Xenon CPU may happen
because of memory shortage or other reasons.

Why not run the normal ppc version?  After all in general there shouldn't
be anything gained using 64bit on powerpc, especially since you have
way less than 4GB ram.  The code size increase from the larger pointers
will probably hurt performance instead.

I believe the ppc64 port has chosen a baseline that requires altivec
which means if you don't have it, stick with plain ppc.

Certainly altivec is a useful feature if you have it so enabling it
makes sense.

It is good or not to adopt these build options, I think that it depends on evaluation of merits and demerits.

It is glad that this subject comes up at the now in front of the release of wheezy,
because the great part of packages of sid will interchange immediately after wheezy release.

Best regards,
Hiroyuki Yamamoto
A75D B285 7050 4BF9 AEDA  91AC 3A10 59C6 5203 04DC

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