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

[David Gibson <dwg@au1.ibm.com>]

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On Wed, May 01, 2013 at 03:52:30PM +0800, Simon Jeons wrote:
> On 05/01/2013 03:47 PM, Aneesh Kumar K.V wrote:
> >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.
> 
> 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?

Complex and debatable at best.  But in any case it's a fixed property
of the CPU, not something that can be changed in software.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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