Re: [Qemu-devel] [PATCH] OSX x86_32 host support

[Jamie Lokier <jamie@shareable.org>]

Alexander Graf wrote:
> Jamie Lokier wrote:
> > Alexander Graf wrote:
> >> I believe the 5% performance hit  
> >> that goes with them is no real problem, as most people should be using  
> >> x86_64 nowadays anyway.
> >
> > *Boggle*!  x86_64 is only a few years old, and cheap low-power x86_64
> > laptops are relatively recent.
> >   
> So you really want to do dynamic retranslation on ancient hardware?

I think you are using 'ancient' to mean what I call 'relatively
recent'.

I bought my current main machine just 1.5 years ago, and there were
_no_ fast 64-bit low power laptop CPUs available at the time.  I
hardly call 1.5 years ancient.  Now there are chips available, but the
overall improvement isn't enough to justify paying to replace a good
1.5 year old machine yet.

> To me emulated systems already feel slow on really recent machines,
> I don't want to go back to something even slower.

No problem with your experience, and you're right about the speed of
course.  I only object to generalising that, to say that nearly
everyone _should_ be using x86_64 now, and (by implication) that
x86_32 is no longer relevant and should get second class support.  It
seems to imply that only people who buy a new computer every 2 years
(at most) use Qemu.  But Qemu is used for lots of different things,
and on lots of machines.  It'll be a few more years before 32-bit Qemu
users are a rarity, imho.

> If you use kqemu there even is near no performance hit at all, which I
> believe is the main use of qemu on i386 anyway.

It's also the main use of Qemu on x86_64, so irrelevant point :-)

> Furthermore x86_64 is _way_ faster, as it provides a lot more
> registers.

For some loads, x86_64 is slower because it uses more memory for
pointers.  But really, the actual relative speed of 32/64 x86 is quite
offtopic.  It's the idea that people "should" use x86_64 with Qemu and
therefore 32-bit x86 should be a second class platform that I object
to.  I don't think it's time for that yet.

> I think the benefit you get from cutting the gcc3 dependency is way more
> important than a major performance hit that people will usually only see
> on the next release of qemu, by which time things have shifted towards
> x86_64 even more.

This I agree with.  Cutting the gcc3 dependency is good, even if it
costs a little in performance.  Clearly, the best performance would
come from a different kind of code generation backend which understood
the host CPU directly, not depending on GCC code fragments at all.

-- Jamie