Re: [PULL 3/9] meson: Disallow 64-bit on 32-bit Xen emulation

[Philippe Mathieu-Daudé <philmd@xxxxxxxxxx>]

On 18/2/25 16:25, Philippe Mathieu-Daudé wrote:
> +Vikram
>
> On 18/2/25 15:10, Andrew Cooper wrote:
> > On 18/02/2025 11:20 am, Philippe Mathieu-Daudé wrote:
> > > Hi,
> > >
> > > Adding Xen community.
> > >
> > > On 8/2/25 21:57, Richard Henderson wrote:
> > > > Require a 64-bit host binary to spawn a 64-bit guest.
> > > >
> > > > Reviewed-by: Thomas Huth <thuth@xxxxxxxxxx>
> > > > Reviewed-by: Philippe Mathieu-Daudé <philmd@xxxxxxxxxx>
> > > > Signed-off-by: Richard Henderson <richard.henderson@xxxxxxxxxx>
> > > > ---
> > > >    meson.build | 9 +++++++--
> > > >    1 file changed, 7 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/meson.build b/meson.build
> > > > index 1af8aeb194..911955cfa8 100644
> > > > --- a/meson.build
> > > > +++ b/meson.build
> > > > @@ -304,9 +304,14 @@ else
> > > >    endif
> > > >    accelerator_targets = { 'CONFIG_KVM': kvm_targets }
> > > >    -if cpu in ['x86', 'x86_64']
> > > > +if cpu == 'x86'
> > > > +  xen_targets = ['i386-softmmu']
> > > > +elif cpu == 'x86_64'
> > > >      xen_targets = ['i386-softmmu', 'x86_64-softmmu']
> > > > -elif cpu in ['arm', 'aarch64']
> > > > +elif cpu == 'arm'
> > > > +  # i386 emulator provides xenpv machine type for multiple
> > > > architectures
> > > > +  xen_targets = ['i386-softmmu']
> > > Is actually someone *testing* this config? I'm having hard time building
> > > it, so am very suspicious about how it runs, and start to wonder if I'm
> > > not just wasting my time (as could be our CI).
> > It was intentional.  I believe it was Stefano (CC'd) who introduced it.
> In the introduction commit, "ARM targets" is used, so apparently both
> 32/64bit were picked deliberately:
>
> ---
> commit aaea616d54317b8a0154adf52303a51da2d8d56f
> Author: Vikram Garhwal <vikram.garhwal@xxxxxxx>
> Date:   Wed Jun 14 17:03:38 2023 -0700
>
>      meson.build: enable xenpv machine build for ARM
>
>      Add CONFIG_XEN for aarch64 device to support build for ARM targets.
>
>      Signed-off-by: Vikram Garhwal <vikram.garhwal@xxxxxxx>
>      Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>
>      Reviewed-by: Alex Bennée <alex.bennee@xxxxxxxxxx>
>
> diff --git a/meson.build b/meson.build
> index 481865bfa97..cfa98e9e25f 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -136,7 +136,7 @@ endif
>   if cpu in ['x86', 'x86_64', 'arm', 'aarch64']
>     # i386 emulator provides xenpv machine type for multiple architectures
>     accelerator_targets += {
> -    'CONFIG_XEN': ['i386-softmmu', 'x86_64-softmmu'],
> +    'CONFIG_XEN': ['i386-softmmu', 'x86_64-softmmu', 'aarch64-softmmu'],
>     }
>   endif
>   if cpu in ['x86', 'x86_64']
> ---
Hmm wrong commit apparently, but the history isn't clear. See:

-- >8 --
commit 3b6b75506de44c5070639943c30a0ad5850f5d02
Author: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Date:   Mon Sep 17 11:59:41 2012 +0200

    configure: factor out list of supported Xen/KVM/HAX targets

    This will be useful when the functions are called, early in the 
configure
    process, to filter out targets that do not support hardware 
acceleration.
    Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

diff --git a/configure b/configure
...
+supported_xen_target() {
+    test "$xen" = "yes" || return 1
+    glob "$1" "*-softmmu" || return 1
+    case "${1%-softmmu}:$cpu" in
+        arm:arm | aarch64:aarch64 | \
+        i386:i386 | i386:x86_64 | x86_64:i386 | x86_64:x86_64)
+            return 0
+        ;;
+    esac
+    return 1
+}
+
 # default parameters
 source_path=$(dirname "$0")
 cpu=""
@@ -6178,46 +6222,22 @@ echo "TARGET_ABI_DIR=$TARGET_ABI_DIR" >> 
$config_target_mak
 if [ "$HOST_VARIANT_DIR" != "" ]; then
     echo "HOST_VARIANT_DIR=$HOST_VARIANT_DIR" >> $config_target_mak
 fi
-case "$target_name" in
-  i386|x86_64)
-    if test "$xen" = "yes" -a "$target_softmmu" = "yes" ; then
-      echo "CONFIG_XEN=y" >> $config_target_mak
-      if test "$xen_pci_passthrough" = yes; then
+
+if supported_xen_target $target; then
+    echo "CONFIG_XEN=y" >> $config_target_mak
+    if test "$xen_pci_passthrough" = yes; then
         echo "CONFIG_XEN_PCI_PASSTHROUGH=y" >> "$config_target_mak"
-      fi
     fi
-    ;;
-  *)
---

Paolo, Alex, was this intentional?

> > Xen uses qemu-system-i386 everywhere because qemu-system-x86_64 doesn't
> > make compatible VMs.  I'm not sure why; I suspect it's bugs in the Xen
> > machine types, but I don't know QEMU well enough to be sure.
> >
> > Another thing that (at least, was) tied to qemu-system-i386 was using
> > Qemu as a XenBlk <-> QCOW adapter, at which point it wasn't even really
> > a system emulator, just a paravirtual disk implementation.
> >
> > This is, AIUI, what ARM wants with the xenpv machine.  If there's a
> > better way to do this, please do say.
> No, I concur.
>
> > Looking through Xen's CI, I can't see any of the ARM builds building
> > QEMU at all.  I think it's quite possible it's not tested any more.
> We only cross-build, see our cross-arm64-xen-only job:
> https://gitlab.com/qemu-project/qemu/-/jobs/9165958873
>
> Note, if it is not clear, the problem I have is to test Xen on
> 32-bit ARM hosts; I don't have any problem with 64-bit ones.
>
> Regards,
>
> Phil.