Subject: Re: [PATCH] PPC: BOOK3S: Disable/Enable TM looking at the ibm, pa-features device tree entry

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Thu, 01 May 2014 20:44:03 +0530

Michael Neuling <[email-masked]> writes:

> Aneesh Kumar K.V <[email-masked]> wrote:
>
>> Runtime disable transactional memory feature looking at pa-features
>> device tree entry. This provides a mechanism to disable TM on P8
>> systems.
>
> What are we actually achieving with this?

PAPR compliance  :) ? Also I wanted to disable guest kernel from doing
TM related save restore. Guest kernel already look at the cpu feature
before doing that. Hence needed a mechanism to disable the feature. 

Things like

static inline void __switch_to_tm(struct task_struct *prev)
{
	if (cpu_has_feature(CPU_FTR_TM)) {
		tm_enable();
		tm_reclaim_task(prev);
	}
}

>
>> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
>> ---
>>  arch/powerpc/kernel/prom.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>> 
>> diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
>> index 668aa4791fd7..537bd7e7db0b 100644
>> --- a/arch/powerpc/kernel/prom.c
>> +++ b/arch/powerpc/kernel/prom.c
>> @@ -161,6 +161,11 @@ static struct ibm_pa_feature {
>>  	{CPU_FTR_NODSISRALIGN, 0, 0,	1, 1, 1},
>>  	{0, MMU_FTR_CI_LARGE_PAGE, 0,	1, 2, 0},
>>  	{CPU_FTR_REAL_LE, PPC_FEATURE_TRUE_LE, 5, 0, 0},
>> +	/*
>> +	 * We should use CPU_FTR_TM_COMP so that if we disable TM, it won't get
>> +	 * enabled via device tree
>> +	 */
>> +	{CPU_FTR_TM_COMP, 0, 0,		22, 0, 0},
>
> What does this do to guests?  Will it turn TM unavailable into an
> illegal instruction?
>

Good suggestion. I guess it should be facility unavailable interrupt ?
I should also make the sure __init_HFSCR only set HFSCR_TM only if the
cpu feature is enabled ?

-aneesh