Re: [PATCH v1] xen/riscv: make calculation of stack address PC-relative

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.03.2023 19:23, Oleksii wrote:
> On Wed, 2023-03-15 at 08:59 +0100, Jan Beulich wrote:
>> On 14.03.2023 21:16, Oleksii wrote:
>>> I checked in Linux binary how 'la' instruction is transformed, and
>>> it
>>> looks like it is translated as I expect to auipc/addi pair:
>>> ffffffe000001066: 00027517 auipc a0,0x27
>>> ffffffe00000106a: f9a50513 addi a0,a0,-102 # ffffffe000028000
>>> <early_pg_dir>
>>>
>>> I checked compiler flags between Xen and Linux. The difference is
>>> in-
>>> fno-PIE (Linux also adds -mabi and -march to AFLAGS):
>>>
>>> 1. Linux build command of head.S: riscv64-linux-gnu-gcc -Wp,-
>>> MD,arch/riscv/kernel/.head.o.d -nostdinc -isystem /usr/lib/gcc-
>>> cross/riscv64-linux-gnu/9/include -I./arch/riscv/include -
>>> I./arch/riscv/include/generated -I./include -
>>> I./arch/riscv/include/uapi
>>> -I./arch/riscv/include/generated/uapi -I./include/uapi -
>>> I./include/generated/uapi -include ./include/linux/kconfig.h -
>>> D__KERNEL__ -D__ASSEMBLY__ -fno-PIE -mabi=lp64 -march=rv64imafdc -c
>>> -o
>>> arch/riscv/kernel/head.o arch/riscv/kernel/head.S
>>>
>>> 2. Xen build command of head.S:riscv64-linux-gnu-gcc -MMD -MP -MF
>>> arch/riscv/riscv64/.head.o.d -D__ASSEMBLY__ -Wa,--noexecstack -
>>> DBUILD_ID -fno-strict-aliasing -Wall -Wstrict-prototypes -
>>> Wdeclaration-
>>> after-statement -Wno-unused-but-set-variable -Wno-unused-local-
>>> typedefs
>>> -O1 -fno-omit-frame-pointer -nostdinc -fno-builtin -fno-common -
>>> Werror
>>> -Wredundant-decls -Wno-pointer-arith -Wvla -pipe -D__XEN__ -include
>>> ./include/xen/config.h -Wa,--strip-local-absolute -g -mabi=lp64 -
>>> I./include -I./arch/riscv/include -march=rv64gc -mstrict-align -
>>> mcmodel=medany - -c arch/riscv/riscv64/head.S -o
>>> arch/riscv/riscv64/head.o
>>
>> Looking into why you see different code generated than I: Nothing in
>> here directs gcc to pass -fpic to gas; in upstream gcc (consistent
>> from gcc7 through gcc12, which are the versions I've checked; the
>> actual range may be wider) there is
>>
>> #define ASM_SPEC "\
>> %(subtarget_asm_debugging_spec) \
>> %{" FPIE_OR_FPIC_SPEC ":-fpic} \
>> ...
>>
>> Can you check whether your gcc passes -fpic to gas even when there's
>> no -fPIC / -fPIE (or alike) on the gcc command line?
> I am not sure that I know how to check specifically if -fpic flag
> passes to gas.
> Could you please tell me?

Just to answer this question here (the other aspect fit better elsewhere):
You can pass -v to gcc to make it report what options it invokes other
tools (including gas) with.

Jan