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

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

David Gibson <dwg@au1.ibm.com> writes:

> On Tue, Apr 30, 2013 at 10:51:00PM +0530, Aneesh Kumar K.V wrote:
>> David Gibson <dwg@au1.ibm.com> writes:
>> 
>> > On Mon, Apr 29, 2013 at 01:07:39AM +0530, Aneesh Kumar K.V wrote:
>> >> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
>> >> 
>> >> 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 <aneesh.kumar@linux.vnet.ibm.com>
>> >> ---
>> >>  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. 

-aneesh