Re: [Qemu-devel] [PATCH v2 1/9] configure: change CONFIG_XEN_BACKEND to be a target property

[Paolo Bonzini <pbonzini@redhat.com>]

On 14/03/2017 00:55, Stefano Stabellini wrote:
> CONFIG_XEN_BACKEND is currently set when the host supports Xen,
> regardless of the chosen targets. As a consequence, Xen backends can be
> enabled even on targets that don't support Xen.
> 
> Fix the issue by setting CONFIG_XEN_BACKEND only for targets that
> support Xen.
> 
> Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
> CC: groug@kaod.org
> CC: groug@kaod.org
> CC: pbonzini@redhat.com
> CC: peter.maydell@linaro.org
> CC: rth@twiddle.net
> CC: stefanha@redhat.com
> ---
>  configure | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/configure b/configure
> index 6c21975..6d8f752 100755
> --- a/configure
> +++ b/configure
> @@ -5442,7 +5442,6 @@ if test "$virglrenderer" = "yes" ; then
>    echo "VIRGL_LIBS=$virgl_libs" >> $config_host_mak
>  fi
>  if test "$xen" = "yes" ; then
> -  echo "CONFIG_XEN_BACKEND=y" >> $config_host_mak
>    echo "CONFIG_XEN_CTRL_INTERFACE_VERSION=$xen_ctrl_version" >> $config_host_mak
>    if test "$xen_pv_domain_build" = "yes" ; then
>      echo "CONFIG_XEN_PV_DOMAIN_BUILD=y" >> $config_host_mak
> @@ -6028,6 +6027,7 @@ case "$target_name" in
>    i386|x86_64)
>      if test "$xen" = "yes" -a "$target_softmmu" = "yes" ; then
>        echo "CONFIG_XEN=y" >> $config_target_mak
> +      echo "CONFIG_XEN_BACKEND=y" >> $config_target_mak
>        if test "$xen_pci_passthrough" = yes; then
>          echo "CONFIG_XEN_PCI_PASSTHROUGH=y" >> "$config_target_mak"
>        fi
> 

This messes up a bit the way xen_nic.o and friends are compiled, I
think, because they are common-obj-y but they are only found when
compiling in the target subdirectories.  So you end up building
x86_64-softmmu/../hw/net/xen_nic.o

If you're unlucky, I believe this can lead to a link failure where a
target is building xen_nic.o, while the other tries to link to a
partially written object file.

I think the files should be changed from
common-obj-$(CONFIG_XEN_BACKEND) to common-obj-$(CONFIG_XEN).  Then you
add to Makefile:

 CONFIG_SOFTMMU := $(if $(filter %-softmmu,$(TARGET_DIRS)),y)
 CONFIG_USER_ONLY := $(if $(filter %-user,$(TARGET_DIRS)),y)
+CONFIG_XEN := $(CONFIG_XEN_BACKEND)
 CONFIG_ALL=y
 -include config-all-devices.mak
 -include config-all-disas.mak

The Makefile change ensures that they are built before descending in the
target-specific directories.  The Makefile.objs change ensures that Xen
backends are not enabled on targets that support Xen.

Paolo