Re: [kvm-unit-tests PATCH] arm64: Compile with -mno-outline-atomics for GCC >= 10

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Drew,

On 7/18/20 2:50 PM, Alexandru Elisei wrote:
> Hi,
>
> On 7/18/20 10:11 AM, Andrew Jones wrote:
>> On Fri, Jul 17, 2020 at 05:47:27PM +0100, Alexandru Elisei wrote:
>>> GCC 10.1.0 introduced the -m{,no-}outline-atomics flags which, according to
>>> man 1 gcc:
>>>
>>> "Enable or disable calls to out-of-line helpers to implement atomic
>>> operations.  These helpers will, at runtime, determine if the LSE
>>> instructions from ARMv8.1-A can be used; if not, they will use the
>>> load/store-exclusive instructions that are present in the base ARMv8.0 ISA.
>>> [..] This option is on by default."
>>>
>>> Unfortunately the option causes the following error at compile time:
>>>
>>> aarch64-linux-gnu-ld -nostdlib -pie -n -o arm/spinlock-test.elf -T /path/to/kvm-unit-tests/arm/flat.lds \
>>> 	arm/spinlock-test.o arm/cstart64.o lib/libcflat.a lib/libfdt/libfdt.a /usr/lib/gcc/aarch64-linux-gnu/10.1.0/libgcc.a lib/arm/libeabi.a arm/spinlock-test.aux.o
>>> aarch64-linux-gnu-ld: /usr/lib/gcc/aarch64-linux-gnu/10.1.0/libgcc.a(lse-init.o): in function `init_have_lse_atomics':
>>> lse-init.c:(.text.startup+0xc): undefined reference to `__getauxval'
>>>
>>> This is happening because we are linking against our own libcflat which
>>> doesn't implement the function __getauxval().
>>>
>>> Disable the use of the out-of-line functions by compiling with
>>> -mno-outline-atomics if we detect a GCC version greater than 10.
>>>
>>> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
>>> ---
>>>
>>> Tested with gcc versions 10.1.0 and 5.4.0 (cross-compilation), 9.3.0
>>> (native).
>>>
>>> I've been able to suss out the reason for the build failure from this
>>> rejected gcc patch [1].
>>>
>>> [1] https://patches.openembedded.org/patch/172460/
>>>
>>>  arm/Makefile.arm64 | 6 ++++++
>>>  1 file changed, 6 insertions(+)
>>>
>>> diff --git a/arm/Makefile.arm64 b/arm/Makefile.arm64
>>> index dfd0c56fe8fb..3223cb966789 100644
>>> --- a/arm/Makefile.arm64
>>> +++ b/arm/Makefile.arm64
>>> @@ -9,6 +9,12 @@ ldarch = elf64-littleaarch64
>>>  arch_LDFLAGS = -pie -n
>>>  CFLAGS += -mstrict-align
>>>  
>>> +# The -mno-outline-atomics flag is only valid for GCC versions 10 and greater.
>>> +GCC_MAJOR_VERSION=$(shell $(CC) -dumpversion 2> /dev/null | cut -f1 -d.)
>>> +ifeq ($(shell expr "$(GCC_MAJOR_VERSION)" ">=" "10"), 1)
>>> +CFLAGS += -mno-outline-atomics
>>> +endif
>> How about this patch instead?
>>
>> diff --git a/Makefile b/Makefile
>> index 3ff2f91600f6..0e21a49096ba 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -17,6 +17,11 @@ DESTDIR := $(PREFIX)/share/kvm-unit-tests/
>>  
>>  .PHONY: arch_clean clean distclean cscope
>>  
>> +# cc-option
>> +# Usage: OP_CFLAGS+=$(call cc-option, -falign-functions=0, -malign-functions=0)
>> +cc-option = $(shell if $(CC) -Werror $(1) -S -o /dev/null -xc /dev/null \
>> +              > /dev/null 2>&1; then echo "$(1)"; else echo "$(2)"; fi ;)
>> +
>>  #make sure env CFLAGS variable is not used
>>  CFLAGS =
>>  
>> @@ -43,12 +48,6 @@ OBJDIRS += $(LIBFDT_objdir)
>>  #include architecture specific make rules
>>  include $(SRCDIR)/$(TEST_DIR)/Makefile
>>  
>> -# cc-option
>> -# Usage: OP_CFLAGS+=$(call cc-option, -falign-functions=0, -malign-functions=0)
>> -
>> -cc-option = $(shell if $(CC) -Werror $(1) -S -o /dev/null -xc /dev/null \
>> -              > /dev/null 2>&1; then echo "$(1)"; else echo "$(2)"; fi ;)
>> -
>>  COMMON_CFLAGS += -g $(autodepend-flags) -fno-strict-aliasing -fno-common
>>  COMMON_CFLAGS += -Wall -Wwrite-strings -Wempty-body -Wuninitialized
>>  COMMON_CFLAGS += -Wignored-qualifiers -Werror
>> diff --git a/arm/Makefile.arm64 b/arm/Makefile.arm64
>> index dfd0c56fe8fb..dbc7524d3070 100644
>> --- a/arm/Makefile.arm64
>> +++ b/arm/Makefile.arm64
>> @@ -9,6 +9,9 @@ ldarch = elf64-littleaarch64
>>  arch_LDFLAGS = -pie -n
>>  CFLAGS += -mstrict-align
>>  
>> +mno_outline_atomics := $(call cc-option, -mno-outline-atomics, "")
>> +CFLAGS += $(mno_outline_atomics)
>> +
>>  define arch_elf_check =
>>  	$(if $(shell ! $(OBJDUMP) -R $(1) >&/dev/null && echo "nok"),
>>  		$(error $(shell $(OBJDUMP) -R $(1) 2>&1)))
>>
>>
>> Thanks,
>> drew
> Looks much better than my version. Do you want me to spin a v2 or do you want to
> send it as a separate patch? If that's the case, I tested the same way I did my
> patch (gcc 10.1.0 and 5.4.0 for cross-compiling, 9.3.0 native):
>
> Tested-by: Alexandru Elisei <alexandru.elisei@arm.com>

Gentle ping regarding this.

Thanks,
Alex
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