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

[Aneesh Kumar K.V]

"Aneesh Kumar K.V" <aneesh.kumar at linux.vnet.ibm.com> writes:

> "Aneesh Kumar K.V" <aneesh.kumar at linux.vnet.ibm.com> writes:
>
>> Michael Neuling <mikey at neuling.org> writes:
>>
>>> Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com> 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 <aneesh.kumar at linux.vnet.ibm.com>
>>>> ---
>>>>  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 ?
>
> I looked at this and I guess we don't need to update HFSCR considering
> that the guest kernel (privileged) access to TM always happen within
> if (cpu_has_feature(CPU_FTR_TM)) conditional block. Also we want to
> disable this per guest and there is no easy way to suggest hypervisor
> that disable TM in HFSCR.
>
> BTW we already do this for guest problme state. We do in guest kernel
>
> 	if (cpu_has_feature(CPU_FTR_TM))
> 		regs->msr |= MSR_TM;
>
> IIUC that should result in facility unavailable interrupt when problem
> state try to access TM ?
>
> I will try to run some test with the patch and update here.

This will actually result in illegal instruction.

-aneesh