powerpc/mm: Fix hash computation function

From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>

The ASM version of hash computation function was truncating the upper bit.
Make the ASM version similar to hpt_hash function. Remove masking vsid bits.
Without this patch, we observed hang during bootup due to not satisfying page
fault request correctly. The fault handler used wrong hash values to update
the HPTE. Hence we kept looping with page fault.

hash_page(ea=000001003e260008, access=203, trap=300 ip=3fff91787134 dsisr 42000000
The computed value of hash 000000000f22f390
update: avpnv=4003e46054003e00, hash=000000000722f390, f=80000006, psize: 2 ...

Reported-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
---
 arch/powerpc/mm/hash_low_64.S |   62 +++++++++++++++++++++++------------------
 1 file changed, 35 insertions(+), 27 deletions(-)

With the fix, the machine can boot up successfully

Tested-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>

于 2013/1/30 13:40, Aneesh Kumar K.V 写道:
> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
>
> The ASM version of hash computation function was truncating the upper bit.
> Make the ASM version similar to hpt_hash function. Remove masking vsid bits.
> Without this patch, we observed hang during bootup due to not satisfying page
> fault request correctly. The fault handler used wrong hash values to update
> the HPTE. Hence we kept looping with page fault.
>
> hash_page(ea=000001003e260008, access=203, trap=300 ip=3fff91787134 dsisr 42000000
> The computed value of hash 000000000f22f390
> update: avpnv=4003e46054003e00, hash=000000000722f390, f=80000006, psize: 2 ...
>
> Reported-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> ---
>  arch/powerpc/mm/hash_low_64.S |   62 +++++++++++++++++++++++------------------
>  1 file changed, 35 insertions(+), 27 deletions(-)
>
> diff --git a/arch/powerpc/mm/hash_low_64.S b/arch/powerpc/mm/hash_low_64.S
> index 5658508..7443481 100644
> --- a/arch/powerpc/mm/hash_low_64.S
> +++ b/arch/powerpc/mm/hash_low_64.S
> @@ -115,11 +115,13 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_1T_SEGMENT)
>  	sldi	r29,r5,SID_SHIFT - VPN_SHIFT
>  	rldicl  r28,r3,64 - VPN_SHIFT,64 - (SID_SHIFT - VPN_SHIFT)
>  	or	r29,r28,r29
> -
> -	/* Calculate hash value for primary slot and store it in r28 */
> -	rldicl	r5,r5,0,25		/* vsid & 0x0000007fffffffff */
> -	rldicl	r0,r3,64-12,48		/* (ea >> 12) & 0xffff */
> -	xor	r28,r5,r0
> +	/*
> +	 * Calculate hash value for primary slot and store it in r28
> +	 * r3 = va, r5 = vsid
> +	 * r0 = (va >> 12) & ((1ul << (28 - 12)) -1)
> +	 */
> +	rldicl	r0,r3,64-12,48
> +	xor	r28,r5,r0		/* hash */
>  	b	4f
>
>  3:	/* Calc vpn and put it in r29 */
> @@ -130,11 +132,12 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_1T_SEGMENT)
>  	/*
>  	 * calculate hash value for primary slot and
>  	 * store it in r28 for 1T segment
> +	 * r3 = va, r5 = vsid
>  	 */
> -	rldic	r28,r5,25,25		/* (vsid << 25) & 0x7fffffffff */
> -	clrldi	r5,r5,40		/* vsid & 0xffffff */
> -	rldicl	r0,r3,64-12,36		/* (ea >> 12) & 0xfffffff */
> -	xor	r28,r28,r5
> +	sldi	r28,r5,25		/* vsid << 25 */
> +	/* r0 =  (va >> 12) & ((1ul << (40 - 12)) -1) */
> +	rldicl	r0,r3,64-12,36
> +	xor	r28,r28,r5		/* vsid ^ ( vsid << 25) */
>  	xor	r28,r28,r0		/* hash */
>
>  	/* Convert linux PTE bits into HW equivalents */
> @@ -407,11 +410,13 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_1T_SEGMENT)
>  	 */
>  	rldicl  r28,r3,64 - VPN_SHIFT,64 - (SID_SHIFT - VPN_SHIFT)
>  	or	r29,r28,r29
> -
> -	/* Calculate hash value for primary slot and store it in r28 */
> -	rldicl	r5,r5,0,25		/* vsid & 0x0000007fffffffff */
> -	rldicl	r0,r3,64-12,48		/* (ea >> 12) & 0xffff */
> -	xor	r28,r5,r0
> +	/*
> +	 * Calculate hash value for primary slot and store it in r28
> +	 * r3 = va, r5 = vsid
> +	 * r0 = (va >> 12) & ((1ul << (28 - 12)) -1)
> +	 */
> +	rldicl	r0,r3,64-12,48
> +	xor	r28,r5,r0		/* hash */
>  	b	4f
>
>  3:	/* Calc vpn and put it in r29 */
> @@ -426,11 +431,12 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_1T_SEGMENT)
>  	/*
>  	 * Calculate hash value for primary slot and
>  	 * store it in r28  for 1T segment
> +	 * r3 = va, r5 = vsid
>  	 */
> -	rldic	r28,r5,25,25		/* (vsid << 25) & 0x7fffffffff */
> -	clrldi	r5,r5,40		/* vsid & 0xffffff */
> -	rldicl	r0,r3,64-12,36		/* (ea >> 12) & 0xfffffff */
> -	xor	r28,r28,r5
> +	sldi	r28,r5,25		/* vsid << 25 */
> +	/* r0 = (va >> 12) & ((1ul << (40 - 12)) -1) */
> +	rldicl	r0,r3,64-12,36
> +	xor	r28,r28,r5		/* vsid ^ ( vsid << 25) */
>  	xor	r28,r28,r0		/* hash */
>
>  	/* Convert linux PTE bits into HW equivalents */
> @@ -752,25 +758,27 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_1T_SEGMENT)
>  	rldicl  r28,r3,64 - VPN_SHIFT,64 - (SID_SHIFT - VPN_SHIFT)
>  	or	r29,r28,r29
>
> -	/* Calculate hash value for primary slot and store it in r28 */
> -	rldicl	r5,r5,0,25		/* vsid & 0x0000007fffffffff */
> -	rldicl	r0,r3,64-16,52		/* (ea >> 16) & 0xfff */
> -	xor	r28,r5,r0
> +	/* Calculate hash value for primary slot and store it in r28
> +	 * r3 = va, r5 = vsid
> +	 * r0 = (va >> 16) & ((1ul << (28 - 16)) -1)
> +	 */
> +	rldicl	r0,r3,64-16,52
> +	xor	r28,r5,r0		/* hash */
>  	b	4f
>
>  3:	/* Calc vpn and put it in r29 */
>  	sldi	r29,r5,SID_SHIFT_1T - VPN_SHIFT
>  	rldicl  r28,r3,64 - VPN_SHIFT,64 - (SID_SHIFT_1T - VPN_SHIFT)
>  	or	r29,r28,r29
> -
>  	/*
>  	 * calculate hash value for primary slot and
>  	 * store it in r28 for 1T segment
> +	 * r3 = va, r5 = vsid
>  	 */
> -	rldic	r28,r5,25,25		/* (vsid << 25) & 0x7fffffffff */
> -	clrldi	r5,r5,40		/* vsid & 0xffffff */
> -	rldicl	r0,r3,64-16,40		/* (ea >> 16) & 0xffffff */
> -	xor	r28,r28,r5
> +	sldi	r28,r5,25		/* vsid << 25 */
> +	/* r0 = (va >> 16) & ((1ul << (40 - 16)) -1) */
> +	rldicl	r0,r3,64-16,40
> +	xor	r28,r28,r5		/* vsid ^ ( vsid << 25) */
>  	xor	r28,r28,r0		/* hash */
>
>  	/* Convert linux PTE bits into HW equivalents */

On Wed, 2013-01-30 at 11:10 +0530, Aneesh Kumar K.V wrote:
> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> 
> The ASM version of hash computation function was truncating the upper bit.
> Make the ASM version similar to hpt_hash function. Remove masking vsid bits.
> Without this patch, we observed hang during bootup due to not satisfying page
> fault request correctly.

Which commit(s) introduced the bug?

cheers

On Mon, 2013-02-04 at 14:29 +1100, Michael Ellerman wrote:
> On Wed, 2013-01-30 at 11:10 +0530, Aneesh Kumar K.V wrote:
> > From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> > 
> > The ASM version of hash computation function was truncating the upper bit.
> > Make the ASM version similar to hpt_hash function. Remove masking vsid bits.
> > Without this patch, we observed hang during bootup due to not satisfying page
> > fault request correctly.
> 
> Which commit(s) introduced the bug?

The bug has been there for ever (well, as far back as git goes without
digging the old monster repo) so may as well date from my original port
of the C hashing code to asm.

However it wasn't per-se a bug until Aneesh 64T support was added in 3.7
since we didn't use the top vsid bits that are masked.

I've merged the patch. My only worry is whether there might have been a
reason for the masking in the first place, ie, do we ever carry VSIDs
with bad bits at the top... though the hash mask should take care of
that in any case (well I hope...)

Cheers,
Ben.