Re: [RFC PATCH] configure: deprecate 32 bit build hosts

[Jakob Bohm]

On 26/09/2019 17:31, Alex Bennée wrote:
> Peter Maydell <address@hidden> writes:
>
> > On Thu, 26 Sep 2019 at 00:31, Alex Bennée <address@hidden> wrote:
> > > The 32 bit hosts are already a second class citizen especially with
> > > support for running 64 bit guests under TCG. We are also limited by
> > > testing as actual working 32 bit machines are getting quite rare in
> > > developers personal menageries. For TCG supporting newer types like
> > > Int128 is a lot harder with 32 bit calling conventions compared to
> > > their larger bit sized cousins. Fundamentally address space is the
> > > most useful thing for the translator to have even for a 32 bit guest a
> > > 32 bit host is quite constrained.
> > >
> > > As far as I'm aware 32 bit KVM users are even less numerous. Even
> > > ILP32 doesn't make much sense given the address space QEMU needs to
> > > manage.
> > For KVM we should wait until the kernel chooses to drop support,
> > I think.
> I can certainly do that - although I'd still like to know who actually
> uses 32 bit kvm support these days.
Note that the hardware support for kvm was apparently only in 64 bit
AMD/IntelCPUs.  I don't know if early Intel Xeons with 64 bit and kvm
support ran faster in 32 bit mode, which would be a good reason to
use a 32 bit host kernel on them these days, but otherwise I see
little reason to treat 32 bit x86 kvm as a priority target.

I don't know what was used to run 32 bit ARM binaries on x86 Android
tablets with 32 bit x86 user space, but if it was qemu, it would have
been TCG, not kvm.

> > > @@ -745,19 +744,22 @@ case "$cpu" in
> > >     ;;
> > >     armv*b|armv*l|arm)
> > >       cpu="arm"
> > > -    supported_cpu="yes"
> > >     ;;
> > I'll leave others to voice opinions about their architectures,
> > but I still have 32-bit arm in my test set for builds, and
> > I'm pretty sure we have users (raspi users, for a start).
> raspi is probably the most common one because of the 32 bit userspace
> they use even though they are running on 64 bit chips.
Please note that not all releases of the raspi boards had 64 bit CPUs.
Raspi 0, 1 and 2 used 32 bit cores (some later production runs of Raspi
2 actually had the Raspi 3 chip).

Then there's the whole space of raspi-lookalike boards like the bananas
(mostly 32 bit A7, some boards with 64 bit A53). LeMaker Etc.

Though I don't currently run qemu on them, I happen to have some older
gen dev boards used for various things, including the BeagleBone open
hardware designs that are all based on a 32 bit Arm A8 core.

Outside of dev boards, there's also the whole theme of running qemu on
Arm based Androidsmartphonesas a way to keep the inner OS separate
from the phone OS.

Enjoy

Jakob
--
Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark.  Direct +45 31 13 16 10
This public discussion message is non-binding and may contain errors.
WiseMo - Remote Service Management for PCs, Phones and Embedded