On 05/23/2019 12:36 PM, Will Deacon wrote:
> [+Daniel and Jean-Philippe]
>
> On Thu, May 23, 2019 at 05:12:00PM +0900, Yoshihiro Shimoda wrote:
>> The following build warning happens on gcc 8.1.0.
>>
>> linux/arch/arm64/include/asm/insn.h: In function 'aarch64_insn_is_ldadd':
>> linux/arch/arm64/include/asm/insn.h:280:257: warning: bitwise comparison always evaluates to false [-Wtautological-compare]
>> __AARCH64_INSN_FUNCS(ldadd, 0x3F20FC00, 0xB8200000)
>>
>> Since the second argument is mask value and compare with the third
>> argument value, the bit 31 is always masked and then this macro is
>> always false. So, this patch fixes the issue.
>>
>> Reported-by: Kuninori Morimoto <kuninori.morimoto.gx@xxxxxxxxxxx>
>> Fixes: 34b8ab091f9ef57a ("bpf, arm64: use more scalable stadd over ldxr / stxr loop in xadd")
>> Tested-by: Kuninori Morimoto <kuninori.morimoto.gx@xxxxxxxxxxx>
>> Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@xxxxxxxxxxx>
>> ---
>> I'm not sure the second argument "0xBF20FC00" is OK or not (we can set
>> to 0xFF20FC00 instead). So, I marked RFC on this patch.
>>
>> arch/arm64/include/asm/insn.h | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/include/asm/insn.h b/arch/arm64/include/asm/insn.h
>> index ec894de..c9e3cdc 100644
>> --- a/arch/arm64/include/asm/insn.h
>> +++ b/arch/arm64/include/asm/insn.h
>> @@ -277,7 +277,7 @@ __AARCH64_INSN_FUNCS(adrp, 0x9F000000, 0x90000000)
>> __AARCH64_INSN_FUNCS(prfm, 0x3FC00000, 0x39800000)
>> __AARCH64_INSN_FUNCS(prfm_lit, 0xFF000000, 0xD8000000)
>> __AARCH64_INSN_FUNCS(str_reg, 0x3FE0EC00, 0x38206800)
>> -__AARCH64_INSN_FUNCS(ldadd, 0x3F20FC00, 0xB8200000)
>> +__AARCH64_INSN_FUNCS(ldadd, 0xBF20FC00, 0xB8200000)
>
> Looking at the ISA encoding, I think that top digit should indeed be 'B',
> but I haven't checked the rest of the instruction.
>
> However, I'm fairly sure we tested this so now I'm a bit worried that I'm
> missing something :/
Hmm, good catch, the mask aka aarch64_insn_is_ldadd() is not used anywhere
in the tree, just the aarch64_insn_get_ldadd_value(). Latter was runtime
tested via BPF JIT as well as through disassembler that it emits ldadd. I
initially had a different mask value than Jean-Philippe, but that was probably
due to confusion on my side. In any case, value should be correct though.
Thanks,
Daniel
