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

[Florian Weimer]

On 10/19/2017 02:04 PM, Tulio Magno Quites Machado Filho wrote:
> Florian Weimer <fweimer at 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.
> 
> Just clarifying: it won't break an application, but abort the transaction,
> which can cause a performance hit.

And in this case, it is better not HTM at all.

>> 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.
> 
> If we change the semantics of this bit, old applications that don't suspend
> transactions won't run on these processors, even if they're safe to run.
> 
> If we adopt the current semantics, only applications that enter in suspend
> state will have to be modified in order to not get a performance regression.

In this light, the trade-off implied by the posted patch seems to be the 
right one.  But the other discussion of an ABI change still makes me a 
bit nervous.

Thanks,
Florian