For system mode, we can rarely support the amount of RAM that
the guest requires. Emulation is restricted to round-robin
mode, which solves many of the atomicity issues, but not those
associated with virtio.  In any case, round-robin does nothing
to help the speed of emulation.

For user mode, most emulation does not succeed at all.  Most
of the time we cannot even load 64-bit non-PIE binaries due
to lack of a 64-bit address space.  Threads are run in
parallel, not round-robin, which means that atomicity
is not handled.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 meson.build | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/meson.build b/meson.build
index 5ca3cc3f34..866b8ce477 100644
--- a/meson.build
+++ b/meson.build
@@ -3176,6 +3176,9 @@ if host_os == 'windows'
   endif
 endif
 
+# Detect host pointer size for the target configuration loop.
+host_long_bits = cc.sizeof('void *') * 8
+
 ########################
 # Target configuration #
 ########################
@@ -3268,11 +3271,18 @@ foreach target : target_dirs
     }
   endif
 
+  config_target += keyval.load('configs/targets' / target + '.mak')
+
   target_kconfig = []
   foreach sym: accelerators
     if sym == 'CONFIG_TCG'
+      # Disallow 64-bit on 32-bit TCG emulation.
+      if host_long_bits < config_target['TARGET_LONG_BITS'].to_int()
+        continue
+      endif
       config_target += { 'CONFIG_TCG_TARGET': 'y' }
     elif target not in accelerator_targets.get(sym, [])
+      # Other accelerators are handled by accelerator_targets.
       continue
     endif
     config_target += { sym: 'y' }
@@ -3286,9 +3296,6 @@ foreach target : target_dirs
     error('No accelerator available for target @0@'.format(target))
   endif
 
-  config_target += keyval.load('configs/targets' / target + '.mak')
-  config_target += { 'TARGET_' + config_target['TARGET_ARCH'].to_upper(): 'y' }
-
   if 'TARGET_NEED_FDT' in config_target and not fdt.found()
     if default_targets
       warning('Disabling ' + target + ' due to missing libfdt')
@@ -3301,6 +3308,7 @@ foreach target : target_dirs
   actual_target_dirs += target
 
   # Add default keys
+  config_target += { 'TARGET_' + config_target['TARGET_ARCH'].to_upper(): 'y' }
   if 'TARGET_BASE_ARCH' not in config_target
     config_target += {'TARGET_BASE_ARCH': config_target['TARGET_ARCH']}
   endif
-- 
2.43.0

Richard Henderson <richard.henderson@linaro.org> writes:

> For system mode, we can rarely support the amount of RAM that
> the guest requires. Emulation is restricted to round-robin
> mode, which solves many of the atomicity issues, but not those
> associated with virtio.  In any case, round-robin does nothing
> to help the speed of emulation.
>
> For user mode, most emulation does not succeed at all.  Most
> of the time we cannot even load 64-bit non-PIE binaries due
> to lack of a 64-bit address space.  Threads are run in
> parallel, not round-robin, which means that atomicity
> is not handled.

I think in itself is not enough, on aarch64.ci.org:

  ./../configure --disable-docs --disable-tools --cross-prefix=arm-linux-gnueabihf-

Should only build 32 bit binaries. Which it does:

  13:52:57 [alex@aarch64:~/l/q/b/arm32.crossbuild] review/deprecate-64-on-32-v2|… + ls qemu-*
  qemu-arm         qemu-keymap        qemu-mipsel       qemu-sh4         qemu-system-i386    qemu-system-ppc      qemu-system-sparc     qemu-xtensa
  qemu-armeb       qemu-m68k          qemu-options.def  qemu-sh4eb       qemu-system-m68k    qemu-system-riscv32  qemu-system-tricore   qemu-xtensaeb
  qemu-hexagon     qemu-microblaze    qemu-or1k         qemu-sparc       qemu-system-mips    qemu-system-rx       qemu-system-xtensa
  qemu-i386        qemu-microblazeel  qemu-ppc          qemu-system-arm  qemu-system-mipsel  qemu-system-sh4      qemu-system-xtensaeb
  qemu-img-cmds.h  qemu-mips          qemu-riscv32      qemu-system-avr  qemu-system-or1k    qemu-system-sh4eb    qemu-version.h

However make check-tcg fails because:

  13:53:09 [alex@aarch64:~/l/q/b/arm32.crossbuild] review/deprecate-64-on-32-v2|… + cat config-host.mak 
  # Automatically generated by configure - do not modify

  all:
  SRC_PATH=/home/alex/lsrc/qemu.git
  TARGET_DIRS=aarch64-linux-user aarch64_be-linux-user alpha-linux-user arm-linux-user armeb-linux-user hexagon-linux-user hppa-linux-user i386-linux-user loongarch64-linux-user m68k-linux-user microblaze-linux-user microblazeel-linux-user mips-linux-user mips64-linux-user mips64el-linux-user mipsel-linux-user mipsn32-linux-user mipsn32el-linux-user or1k-linux-user ppc-linux-user ppc64-linux-user ppc64le-linux-user riscv32-linux-user riscv64-linux-user s390x-linux-user sh4-linux-user sh4eb-linux-user sparc-linux-user sparc32plus-linux-user sparc64-linux-user x86_64-linux-user xtensa-linux-user xtensaeb-linux-user aarch64-softmmu alpha-softmmu arm-softmmu avr-softmmu hppa-softmmu i386-softmmu loongarch64-softmmu m68k-softmmu microblaze-softmmu microblazeel-softmmu mips-softmmu mips64-softmmu mips64el-softmmu mipsel-softmmu or1k-softmmu ppc-softmmu ppc64-softmmu riscv32-softmmu riscv64-softmmu rx-softmmu s390x-softmmu sh4-softmmu sh4eb-softmmu sparc-softmmu sparc64-softmmu tricore-softmmu x86_64-softmmu xtensa-softmmu xtensaeb-softmmu
  GDB=/usr/bin/gdb-multiarch
  RUNC=docker
  SUBDIRS= pc-bios/optionrom pc-bios/s390-ccw
  PYTHON=/home/alex/lsrc/qemu.git/builds/arm32.crossbuild/pyvenv/bin/python3 -B
  MKVENV_ENSUREGROUP=/home/alex/lsrc/qemu.git/builds/arm32.crossbuild/pyvenv/bin/python3 -B /home/alex/lsrc/qemu.git/python/scripts/mkvenv.py ensuregroup  --online
  GENISOIMAGE=/usr/bin/genisoimage
  MESON=/home/alex/lsrc/qemu.git/builds/arm32.crossbuild/pyvenv/bin/meson
  NINJA=/usr/bin/ninja
  EXESUF=
  CONFIG_DEFAULT_TARGETS=y
  TCG_TESTS_TARGETS= aarch64-linux-user arm-linux-user i386-linux-user mips64el-linux-user mipsel-linux-user riscv64-linux-user s390x-linux-user aarch64-softmmu arm-softmmu i386-softmmu riscv64-softmmu s390x-softmmu

So possible TCG_TESTS_TARGET needs to be merged with the meson code?
Also do we still use TARGET_DIRS?

>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  meson.build | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/meson.build b/meson.build
> index 5ca3cc3f34..866b8ce477 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -3176,6 +3176,9 @@ if host_os == 'windows'
>    endif
>  endif
>  
> +# Detect host pointer size for the target configuration loop.
> +host_long_bits = cc.sizeof('void *') * 8
> +
>  ########################
>  # Target configuration #
>  ########################
> @@ -3268,11 +3271,18 @@ foreach target : target_dirs
>      }
>    endif
>  
> +  config_target += keyval.load('configs/targets' / target + '.mak')
> +
>    target_kconfig = []
>    foreach sym: accelerators
>      if sym == 'CONFIG_TCG'
> +      # Disallow 64-bit on 32-bit TCG emulation.
> +      if host_long_bits < config_target['TARGET_LONG_BITS'].to_int()
> +        continue
> +      endif
>        config_target += { 'CONFIG_TCG_TARGET': 'y' }
>      elif target not in accelerator_targets.get(sym, [])
> +      # Other accelerators are handled by accelerator_targets.
>        continue
>      endif
>      config_target += { sym: 'y' }
> @@ -3286,9 +3296,6 @@ foreach target : target_dirs
>      error('No accelerator available for target @0@'.format(target))
>    endif
>  
> -  config_target += keyval.load('configs/targets' / target + '.mak')
> -  config_target += { 'TARGET_' + config_target['TARGET_ARCH'].to_upper(): 'y' }
> -
>    if 'TARGET_NEED_FDT' in config_target and not fdt.found()
>      if default_targets
>        warning('Disabling ' + target + ' due to missing libfdt')
> @@ -3301,6 +3308,7 @@ foreach target : target_dirs
>    actual_target_dirs += target
>  
>    # Add default keys
> +  config_target += { 'TARGET_' + config_target['TARGET_ARCH'].to_upper(): 'y' }
>    if 'TARGET_BASE_ARCH' not in config_target
>      config_target += {'TARGET_BASE_ARCH': config_target['TARGET_ARCH']}
>    endif

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro

On 03/02/2025 04.18, Richard Henderson wrote:
> For system mode, we can rarely support the amount of RAM that
> the guest requires. Emulation is restricted to round-robin
> mode, which solves many of the atomicity issues, but not those
> associated with virtio.  In any case, round-robin does nothing
> to help the speed of emulation.
> 
> For user mode, most emulation does not succeed at all.  Most
> of the time we cannot even load 64-bit non-PIE binaries due
> to lack of a 64-bit address space.  Threads are run in
> parallel, not round-robin, which means that atomicity
> is not handled.
> 
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>   meson.build | 14 +++++++++++---
>   1 file changed, 11 insertions(+), 3 deletions(-)

Reviewed-by: Thomas Huth <thuth@redhat.com>