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	doc: Add info for building Xen target with Clang
When build Xen target with Clang, the linker reports failure. This patch adds the related info in the documentation as a known issue and gives details for how to dismiss the building failure with Clang. Signed-off-by: Leo Yan <leo.yan@linaro.org>
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							@ -74,3 +74,39 @@ simplified with a simple wrapper script - saved as
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    #!/bin/sh
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    exec clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd "$@"
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Known Issues
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------------
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When build U-boot for `xenguest_arm64_defconfig` target, it reports linkage
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error:
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.. code-block:: bash
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    aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `do_hypervisor_callback':
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    /home/leoy/Dev2/u-boot/drivers/xen/hypervisor.c:188: undefined reference to `__aarch64_swp8_acq_rel'
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    aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `synch_test_and_set_bit':
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    /home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:40: undefined reference to `__aarch64_ldset1_acq_rel'
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    aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `synch_test_and_clear_bit':
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    /home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:28: undefined reference to `__aarch64_ldclr1_acq_rel'
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    aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `synch_test_and_set_bit':
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    /home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:40: undefined reference to `__aarch64_ldset1_acq_rel'
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    aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `synch_test_and_clear_bit':
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    /home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:28: undefined reference to `__aarch64_ldclr1_acq_rel'
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    aarch64-linux-gnu-ld.bfd: drivers/xen/events.o: in function `synch_test_and_clear_bit':
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    /home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:28: undefined reference to `__aarch64_ldclr1_acq_rel'
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    aarch64-linux-gnu-ld.bfd: drivers/xen/events.o: in function `synch_test_and_set_bit':
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    /home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:40: undefined reference to `__aarch64_ldset1_acq_rel'
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    aarch64-linux-gnu-ld.bfd: drivers/xen/gnttab.o: in function `gnttab_end_access':
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    /home/leoy/Dev2/u-boot/drivers/xen/gnttab.c:109: undefined reference to `__aarch64_cas2_acq_rel'
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    Segmentation fault
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To fix the failure, we need to append option `-mno-outline-atomics` in Clang
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command to not generate local calls to out-of-line atomic operations:
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.. code-block:: bash
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    make HOSTCC=clang xenguest_arm64_defconfig
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    make HOSTCC=clang CROSS_COMPILE=aarch64-linux-gnu- \
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         CC="clang -target aarch64-linux-gnueabi -mno-outline-atomics" -j8
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