Re: xen bits broke x32 build

[Peter Maydell <peter.maydell@linaro.org>]

On Tue, 11 Apr 2023 at 12:57, Michael Tokarev <mjt@tls.msk.ru> wrote:
>
> 01.04.2023 11:40, Michael Tokarev wrote:
> > After bringing in xen guest support, qemu fails to build on x32:
> >
> > target/i386/kvm/xen-emu.c:876:5: note: in expansion of macro ‘qemu_build_assert’
> >    876 |     qemu_build_assert(sizeof(struct vcpu_info) == 64);
> >        |     ^~~~~~~~~~~~~~~~~
> >
> > This one should be easy to fix, but I wonder if there are other issues
> > with x32 exists..
>
> Ok, I took a look at how to disable this new XEN stuff on x32.
>
> It is the commit 820c1aba519bd072ac71c754733f6c86d8b4309 "xen: add
> CONFIG_XEN_BUS and CONFIG_XEN_EMU options for Xen emulation" adding
> this construct to hw/i386/Kconfig:
>
> config XEN_EMU
>      bool
>      default y
>      depends on KVM && (I386 || X86_64)
>
> Since meson does not know about x32, and while ./conifgure does, it
> is not propagated to meson, and sure not propagated to Kconfig too,
> there's some more work needed to disable XEN_EMU on x32.

Frankly I would prefer to just say "we don't support x32".
It's a weird non-standard configuration that as far as I'm
aware is very little used. Its stats in the debian
popularity-contest graphs peaked at 18 users in 2017, and
have mostly been fluctuating between 1 and 3 for the last
couple of years:
https://popcon.debian.org/stat/sub-x32.png

We're currently planning to deprecate-and-drop 32-bit x86
hosts, which are much more widely used than this. I see
no reason why we should care about this oddball failed
experiment of an ABI...

thanks
-- PMM