Subject: Re: [PATCH -V7 18/18] powerpc: Update tlbie/tlbiel as per ISA doc

From: Simon Jeons <[email-masked]>
Date: Wed, 01 May 2013 15:52:30 +0800

On 05/01/2013 03:47 PM, Aneesh Kumar K.V wrote:
> David Gibson <[email-masked]> writes:
>
>> On Tue, Apr 30, 2013 at 10:51:00PM +0530, Aneesh Kumar K.V wrote:
>>> David Gibson <[email-masked]> writes:
>>>
>>>> On Mon, Apr 29, 2013 at 01:07:39AM +0530, Aneesh Kumar K.V wrote:
>>>>> From: "Aneesh Kumar K.V" <[email-masked]>
>>>>>
>>>>> Encode the actual page correctly in tlbie/tlbiel. This make sure we handle
>>>>> multiple page size segment correctly.
>>>> As mentioned in previous comments, this commit message needs to give
>>>> much more detail about what precisely the existing implementation is
>>>> doing wrong.
>>>>
>>>>> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
>>>>> ---
>>>>>   arch/powerpc/mm/hash_native_64.c | 32 ++++++++++++++++++++++++++++++--
>>>>>   1 file changed, 30 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/arch/powerpc/mm/hash_native_64.c b/arch/powerpc/mm/hash_native_64.c
>>>>> index bb920ee..6a2aead 100644
>>>>> --- a/arch/powerpc/mm/hash_native_64.c
>>>>> +++ b/arch/powerpc/mm/hash_native_64.c
>>>>> @@ -61,7 +61,10 @@ static inline void __tlbie(unsigned long vpn, int psize, int apsize, int ssize)
>>>>>   
>>>>>   	switch (psize) {
>>>>>   	case MMU_PAGE_4K:
>>>>> +		/* clear out bits after (52) [0....52.....63] */
>>>>> +		va &= ~((1ul << (64 - 52)) - 1);
>>>>>   		va |= ssize << 8;
>>>>> +		va |= mmu_psize_defs[apsize].sllp << 6;
>>>>>   		asm volatile(ASM_FTR_IFCLR("tlbie %0,0", PPC_TLBIE(%1,%0), %2)
>>>>>   			     : : "r" (va), "r"(0), "i" (CPU_FTR_ARCH_206)
>>>>>   			     : "memory");
>>>>> @@ -69,9 +72,20 @@ static inline void __tlbie(unsigned long vpn, int psize, int apsize, int ssize)
>>>>>   	default:
>>>>>   		/* We need 14 to 14 + i bits of va */
>>>>>   		penc = mmu_psize_defs[psize].penc[apsize];
>>>>> -		va &= ~((1ul << mmu_psize_defs[psize].shift) - 1);
>>>>> +		va &= ~((1ul << mmu_psize_defs[apsize].shift) - 1);
>>>>>   		va |= penc << 12;
>>>>>   		va |= ssize << 8;
>>>>> +		/* Add AVAL part */
>>>>> +		if (psize != apsize) {
>>>>> +			/*
>>>>> +			 * MPSS, 64K base page size and 16MB parge page size
>>>>> +			 * We don't need all the bits, but rest of the bits
>>>>> +			 * must be ignored by the processor.
>>>>> +			 * vpn cover upto 65 bits of va. (0...65) and we need
>>>>> +			 * 58..64 bits of va.
>>>> I can't understand what this comment is saying.  Why do we need to do
>>>> something different in the psize != apsize case?
>>>>
>>>>> +			 */
>>>>> +			va |= (vpn & 0xfe);
>>>>> +		}
>>> That is as per ISA doc. It says if base page size == actual page size,
>>> (RB)56:62 must be zeros, which must be ignored by the processor.
>>> Otherwise it should be filled with the selected bits of VA as explained above.
>> What you've just said here makes much more sense than what's written
>> in the comment in the code.
>>
>>> We only support MPSS with base page size = 64K and actual page size = 16MB.
>> Is that actually relevant to this code though?
> In a way yes. The number of bits we we select out of VA depends on the
> base page size and actual page size. We have a math around that
> documented in ISA. Now since we support only 64K and 16MB we can make it
> simpler by only selecting required bits and not making it a
> function. But then it is also not relevant to the code in that ISA also
> state other bits in (RB)56:62 must be zero. I wanted to capture both the
> details in the comment.

What's the benefit of ppc use hash-based page table instead of 
tree-based page table? If you said that hash is faster, could x86 change 
to it?

>
> -aneesh
>
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