Re: [PATCH 3/4] powerpc/powernv: Enable TM without suspend if possible

["Tulio Magno Quites Machado Filho" <tuliom@linux.vnet.ibm.com>]

Forwarding some comments from Adhemerval sent to libc-alpha [1]...

Adhemerval Zanella <adhemerval.zanella@linaro.org> writes:
>Florian Weimer <fweimer@redhat.com> writes:
>
>> On 10/12/2017 12:17 PM, Michael Ellerman wrote:
>>> +	pr_info("Enabling TM (Transactional Memory) with Suspend Disabled\n");
>>> +	cur_cpu_spec->cpu_features |= CPU_FTR_TM;
>>> +	cur_cpu_spec->cpu_user_features2 |= PPC_FEATURE2_HTM_NO_SUSPEND;
>>> +	tm_suspend_disabled = true;
>>
>> This doesn't look right because if suspend is not available, you need to 
>> clear the original PPC_FEATURE2_HTM flag because the semantics are not 
>> right, so that applications can use fallback code.  Otherwise, 
>> applications may incorrectly select the HTM code and break if running on 
>> a system which supports HTM, but without the suspend state.
>>
>> The new flag should say that HTM is supported, but without the suspend 
>> state, and it should be always set if PPC_FEATURE2_HTM is set.
>
> Will it also change TEXARS with the abort information?

It should, with a permanent error cause so that old applications entering
suspended state can adopt another technique.
Michael, could you clarify if this is indeed happening, please?

> I completely agree with Florian here, this is as *ABI* change
> and the kernel need to advertise a different TM ABI instead
> of as an extension.

Adhemerval, could you elaborate which problems you're foreseeing, please?

-- 
Tulio Magno

[1] https://sourceware.org/ml/libc-alpha/2017-10/msg00893.html