Re: Deprecate 32-bit hosts? (was: Re: [PULL 14/14] hw/arm/aspeed: Add Fuji machine type)

[Daniel P . Berrangé]

On Wed, Sep 15, 2021 at 10:51:56AM +0200, Philippe Mathieu-Daudé wrote:
> On 9/15/21 10:37 AM, Daniel P. Berrangé wrote:
> > On Wed, Sep 15, 2021 at 09:42:48AM +0200, Thomas Huth wrote:
> >> On 14/09/2021 17.22, Richard Henderson wrote:
> >>> On 9/14/21 5:26 AM, Peter Maydell wrote:
> >>>> (2) RAM blocks should have a length that fits inside a
> >>>>      signed 32-bit type on 32-bit hosts (at least I assume this
> >>>>      is where the 2047MB limit is coming from; in theory this ought
> >>>>      to be improveable but auditing the code for mishandling of
> >>>>      RAMblock sizes to ensure we weren't accidentally stuffing
> >>>>      their size into a signed 'long' somewhere would be kind
> >>>>      of painful)
> >>>
> >>> Recalling that the win64 abi model is p64, i.e. 'long' is still 32-bit
> >>> while pointers are 64-bit, how close do we think we are to this being
> >>> fixed already?
> >>>
> >>>> Even if we did fix (2) we'd need to compromise on (3)
> >>>> sometimes still -- if a board has 4GB of RAM that's
> >>>> not going to fit in 32 bits regardless. But we would be
> >>>> able to let boards with 2GB have 2GB.
> >>>
> >>> I'm not opposed to deprecating 32-bit hosts...  ;-)
> >>
> >> I think we should consider this again, indeed. Plain 32-bit CPUs are quite
> >> seldom these days, aren't they? And I think we urgently need to decrease 
> >> the
> >> amount of things that we have to test and maintain in our CI and developer
> >> branches... So is there still a really really compelling reason to keep
> >> 32-bit host support alive?
> > 
> > I think it probably depends on the architecture to some extent.
> > 
> > i386 is possibly getting rare enough to consider dropping, though
> > IIUC, KVM in the kernel still supports it.  Would feel odd to drop
> > it in QEMU if the kernel still thinks it is popular enough to keep
> > KVM support.
> > 
> > armv7 feels like it is relatively common as 64-bit didn't arrive
> > in widespread use until relatively recent times compared to x86_64.
> > KVM dropped armv7, but then hardware for that was never widespread,
> > so armv7 was always TCG dominated
> > 
> > Other 32-bit arches were/are always rare.
> 
> While I could understand there are rare uses of system emulation on
> 32-bit hosts, I still believe user-emulation is used, but would like
> to be proven to the contrary. With that in mind, I'm not sure removing
> sysemu on 32-bit hosts is worthful. Maybe we should ask distribution
> maintainers first, then eventually poll the community? Or start with
> a deprecation warning?

Well Debian still supports arm7, i686, mips officially, and several
more unofficially, so that's an easy answer from that side.

Fedora only has arm7, having dropped i686 a while ago.

I don't have insight into usage of QEMU on any platforms breakdown
though.

Regards,
Daniel
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