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

From: Daniel P . Berrangé
Subject: Re: [RFC PATCH] configure: deprecate 32 bit build hosts
Date: Mon, 30 Sep 2019 10:25:42 +0100
User-agent: Mutt/1.12.1 (2019-06-15)

On Thu, Sep 26, 2019 at 10:11:05AM -0700, Richard Henderson wrote:
> On 9/26/19 12:50 AM, Peter Maydell wrote:
> > 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.
> Agreed.  I think this discussion should be more about TCG.
> >> @@ -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).
> I'd really like to know what raspi users might be using qemu for.  Depending 
> on
> that answer, perhaps it would be sufficient for arm32 tcg to only support
> 32-bit guests.

I asked on the Fedora development lists for feedback on the idea of
dropping QEMU 32-bit host support


The response was rather underwhealming to say the least, with only one
person explicitly expressing a desire for QEMU to keep 32-bit host
support for armv7l.

The main interesting thing I learnt was that even with 64-bit Raspberry
Pi hardware, it can be desirable to run 32-bit Raspberry Pi supporting
distro, supposedly for performance benefits.

> For context, the discussion that Alex and I were having was about int128_t, 
> and
> how to support that directly in tcg (especially to/from helpers), and how that
> might be vastly easier if we didn't have to consider 32-bit hosts.

I know nothing about TCG internals, but Is it viable to implement int128_t
support only in 64-bit host, leaving 32-bit hosts without it ? Or is this
really a blocking item that is holding back 64-bit host features.

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