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 <alex.ben...@linaro.org> 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 https://lists.fedoraproject.org/archives/list/de...@lists.fedoraproject.org/thread/TPAVIC6YANGP2NK4RUOP7OCIOIFIOV3A/ 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. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
