Re: [Qemu-devel] issue: linking 64bit glib when building for cpu=i386

[John Snow <jsnow@redhat.com>]

On 08/22/2014 05:05 AM, Stefan Hajnoczi wrote:
> On Fri, Aug 22, 2014 at 9:20 AM, Daniel P. Berrange <berrange@redhat.com> wrote:
>> On Thu, Aug 21, 2014 at 08:10:54PM -0400, John Snow wrote:
>>> I was running a series of tests on 32 and 64 bit hosts to test for
>>> endianness and variable width issues when I noticed that I couldn't properly
>>> perform a build of "make check" against a 32bit target from a 64bit host:
>>>
>>> ../../configure --cpu=i386 && make -j4 && make check
>>>
>>> This produces some warnings in tests-cutils about overflowing variables that
>>> are of type guint64. It's been mentioned on the mailing lists before,
>>> actually: http://lists.gnu.org/archive/html/qemu-devel/2014-05/msg00452.html
>>>
>>> The problem is that guint64 is being aliased against "unsigned long", which
>>> is only 4 bytes instead of the implied 8. This occurs because we link
>>> against the 64bit headers for glib instead of the 32bit ones when we're
>>> building for i386 from an x86_64 host.
>>>
>>> Our include flags wind up looking like: -I/usr/include/glib-2.0 but
>>> -I/usr/lib64/glib-2.0/include
>>>
>>> I was discussing the problem with Stefan:
>>>
>>> On 08/21/2014 05:03 AM, Stefan Hajnoczi wrote:
>>>> The problem is that pkg-config uses libdir=/usr/lib64 by default on
>>>> amd64 hosts.  It doesn't know that gcc -m32 is being used.
>>>>
>>>> This results in glib's 64-bit headers being used where guint64 is just
>>>> unsigned long.  On 32-bit hosts this is incorrect.
>>>>
>>>> Two workarounds:
>>>>
>>>> 1. yum install pkgconfig.i686 and run it instead of pkgconfig.x86_64
>>>>
>>>> 2. Use the pkg-config --define-variable libdir=/usr/lib option
>>>>
>>>> You can set PKG_CONFIG=path/to/pkg-config.i686 on QEMU's ./configure
>>>> command-line.
>>>>
>>>> This is all distro-specific :(.  Any other solutions?
>>>>
>>>> Stefan
>>>>
>>>
>>> I am not extremely well versed in configure or pkg-config ninjutsu, but I
>>> must imagine that the ARCH/cpu variables we are setting in configure could
>>> help us know to call the 32bit pkg-config instead of the native 64bit
>>> version and fix this issue.
>>>
>>> Does anyone have any good ideas? Surely other projects must have run into
>>> this elsewhere.
>>
>> Distros will install pkg-config .pc files for non-native architectures
>> in a different location normally. The supported / recommended way to
>> tell pkg-config to look in these alternative dirs is to set the env
>> variable  PKG_CONFIG_LIBDIR. This replaces the built-in default search
>> directory that looks for native.
>>
>> So on a Fedora / RHELL system, to make pkg-config use 32-bit libs you
>> want to set
>>
>>     PKG_CONFIG_LIBDIR=/usr/lib/pkgconfig
>>
>> which replaces the default location of /usr/lib64/pkgconfig.
>
> Nice, and Paolo sent me an automated way of doing that:
>
> On Fri, Aug 22, 2014 at 12:14:28AM +0200, Paolo Bonzini wrote:
>> You need to set PKG_CONFIG_LIBDIR to
>>
>> /usr/lib/$MULTILIBDIR/pkgconfig
>>
>> where MULTILIBDIR is
>>
>>      if $CC -print-multiarch >/dev/null 2>&1; then
>>        MULTILIBDIR=`$CC -print-multiarch $CFLAGS $CPPFLAGS`
>>      fi
>>      if test -z "$MULTILIBDIR"; then
>>        MULTILIBDIR=`$CC --print-multi-os-directory $CFLAGS $CPPFLAGS`
>>      fi
>>
>> This will point at /usr/lib/pkgconfig/glib-2.0.pc instead of
>> /usr/lib64/pkgconfig/glib-2.0.pc
>
> I tested that it works.
>
> Stefan
>

A barrier into introducing this into the configuration script is that 
clang, used on OSX hosts, does not support these print flags.

Even worse, clang --print-multi-os-directory produces "x86_64" on Fedora 
20, which will lead to an erroneous configuration.

So we'll have to look into another way to support cross-compilation for 
i386 on x86_64 to avoid script breakage on other platforms.

Any other ideas? It looks as if there have been proposals in the past to 
add some multi-arch awareness into pkg-config, but they haven't gone 
anywhere. So some tool needs to tell pkg-config where to look, but clang 
is apparently not up to the job.

Where does gcc/g++ find out about the lib directories? Or are they 
baked-in per distro?

Any ideas?

--j