Re: [PATCH] configure: define CONFIG_XEN when Xen is enabled

[Philippe Mathieu-Daudé <philmd@xxxxxxxxxx>]

On 7/28/20 11:27 AM, Peter Maydell wrote:
> On Tue, 28 Jul 2020 at 10:19, Paul Durrant <paul@xxxxxxx> wrote:
>>
>> From: Paul Durrant <pdurrant@xxxxxxxxxx>
>>
>> The recent commit da278d58a092 "accel: Move Xen accelerator code under
>> accel/xen/" introduced a subtle semantic change, making xen_enabled() always
>> return false unless CONFIG_XEN is defined prior to inclusion of sysemu/xen.h,
>> which appears to be the normal case. This causes various use-cases of QEMU
>> with Xen to break.
>>
>> This patch makes sure that CONFIG_XEN is defined if --enable-xen is passed
>> to configure.
>>
>> Fixes: da278d58a092 ("accel: Move Xen accelerator code under accel/xen/")
>> Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
>> ---
>> Cc: "Philippe Mathieu-Daudé" <philmd@xxxxxxxxxx>
>> Cc: Laurent Vivier <laurent@xxxxxxxxx>
>> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>> Cc: Anthony Perard <anthony.perard@xxxxxxxxxx>
>> ---
>>  configure | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/configure b/configure
>> index 2acc4d1465..f1b9d129fd 100755
>> --- a/configure
>> +++ b/configure
>> @@ -7434,6 +7434,7 @@ if test "$virglrenderer" = "yes" ; then
>>    echo "VIRGL_LIBS=$virgl_libs" >> $config_host_mak
>>  fi
>>  if test "$xen" = "yes" ; then
>> +  echo "CONFIG_XEN=y" >> $config_host_mak
>>    echo "CONFIG_XEN_BACKEND=y" >> $config_host_mak
>>    echo "CONFIG_XEN_CTRL_INTERFACE_VERSION=$xen_ctrl_version" >> 
>> $config_host_mak
>>  fi
> 
> Configure already defines CONFIG_XEN as a target-specific
> config define in config-target.mak for the specific targets
> that Xen will work for (ie if you build --enable-xen for
> x86_64-softmmu and ppc64-softmmu then CONFIG_XEN is set for
> the former and not the latter). This patch makes it a
> build-wide config setting by putting it in config-host.mak.
> 
> We should figure out which of those two is correct and do
> just one of them, not do both at the same time.
> 
> Since CONFIG_HAX, CONFIG_KVM and other accelerator-type
> config defines are also per-target, I suspect that the
> correct fix for this bug is not in configure but elsewhere.

This might come from this change:

-#include "hw/xen/xen.h"
+#include "sysemu/xen.h"

Before Xen was target-specific, now it is a target-agnostic accelerator.

However in include/qemu/osdep.h we have:

 30 #include "config-host.h"
 31 #ifdef NEED_CPU_H
 32 #include "config-target.h"
 33 #else
 34 #include "exec/poison.h"
 35 #endif

CONFIG_XEN is generated in "config-target.h" (target-specific),
so target-agnostic code always has it undefined.

I'd rather uninline xen_enabled() but I'm not sure this has perf
penalties. Paolo is that OK to uninline it?

Phil.