linux-kernel - Re: [RFC v2 10/12] powerpc: Read AMR only if pkey-violation caused the exception.

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Message-Id: <d403fa09-4eaa-a8ff-26f6-caf407538253@linux.vnet.ibm.com>
Date:   Tue, 20 Jun 2017 12:16:40 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Ram Pai <linuxram@...ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, dave.hansen@...el.com,
        paulus@...ba.org, aneesh.kumar@...ux.vnet.ibm.com,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC v2 10/12] powerpc: Read AMR only if pkey-violation caused
 the exception.

On 06/19/2017 11:29 PM, Ram Pai wrote:
> On Mon, Jun 19, 2017 at 09:06:13PM +1000, Michael Ellerman wrote:
>> Ram Pai <linuxram@...ibm.com> writes:
>>
>>> Signed-off-by: Ram Pai <linuxram@...ibm.com>
>>> ---
>>>  arch/powerpc/kernel/exceptions-64s.S | 16 ++++++++++------
>>>  1 file changed, 10 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
>>> index 8db9ef8..a4de1b4 100644
>>> --- a/arch/powerpc/kernel/exceptions-64s.S
>>> +++ b/arch/powerpc/kernel/exceptions-64s.S
>>> @@ -493,13 +493,15 @@ EXC_COMMON_BEGIN(data_access_common)
>>>  	ld	r12,_MSR(r1)
>>>  	ld	r3,PACA_EXGEN+EX_DAR(r13)
>>>  	lwz	r4,PACA_EXGEN+EX_DSISR(r13)
>>> +	std	r3,_DAR(r1)
>>> +	std	r4,_DSISR(r1)
>>>  #ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
>>> +	andis.  r0,r4,DSISR_KEYFAULT@h /* save AMR only if its a key fault */
>>> +	beq+	1f
>>
>> This seems to be incremental on top of one of your other patches.
>>
>> But I don't see why, can you please just squash this into whatever patch
>> adds this code in the first place.
> 
> It was an optimization added later. But yes it can be squashed into an
> earlier patch.

Could you please explain what is the optimization this achieves ?