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powerpc/mm: Update VSID allocation documentation

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This update the proto-VSID and VSID scramble related information
to be more generic by using names instead of current values.

Reviewed-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
This commit is contained in:
Aneesh Kumar K.V 2012-09-10 02:52:56 +00:00 committed by Benjamin Herrenschmidt
parent 048ee0993e
commit f033d659c3
2 changed files with 22 additions and 26 deletions
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40
arch/powerpc/include/asm/mmu-hash64.h
View file

@ -324,51 +324,45 @@ extern void slb_set_size(u16 size);
#endif /* __ASSEMBLY__ */
/*
* VSID allocation
* VSID allocation (256MB segment)
*
* We first generate a 36-bit "proto-VSID". For kernel addresses this
* is equal to the ESID, for user addresses it is:
* (context << 15) | (esid & 0x7fff)
* We first generate a 38-bit "proto-VSID". For kernel addresses this
* is equal to the ESID | 1 << 37, for user addresses it is:
* (context << USER_ESID_BITS) | (esid & ((1U << USER_ESID_BITS) - 1)
*
* The two forms are distinguishable because the top bit is 0 for user
* addresses, whereas the top two bits are 1 for kernel addresses.
* Proto-VSIDs with the top two bits equal to 0b10 are reserved for
* now.
* This splits the proto-VSID into the below range
* 0 - (2^(CONTEXT_BITS + USER_ESID_BITS) - 1) : User proto-VSID range
* 2^(CONTEXT_BITS + USER_ESID_BITS) - 2^(VSID_BITS) : Kernel proto-VSID range
*
* We also have CONTEXT_BITS + USER_ESID_BITS = VSID_BITS - 1
* That is, we assign half of the space to user processes and half
* to the kernel.
*
* The proto-VSIDs are then scrambled into real VSIDs with the
* multiplicative hash:
*
* VSID = (proto-VSID * VSID_MULTIPLIER) % VSID_MODULUS
* where VSID_MULTIPLIER = 268435399 = 0xFFFFFC7
* VSID_MODULUS = 2^36-1 = 0xFFFFFFFFF
*
* This scramble is only well defined for proto-VSIDs below
* 0xFFFFFFFFF, so both proto-VSID and actual VSID 0xFFFFFFFFF are
* reserved. VSID_MULTIPLIER is prime, so in particular it is
* VSID_MULTIPLIER is prime, so in particular it is
* co-prime to VSID_MODULUS, making this a 1:1 scrambling function.
* Because the modulus is 2^n-1 we can compute it efficiently without
* a divide or extra multiply (see below).
*
* This scheme has several advantages over older methods:
*
* - We have VSIDs allocated for every kernel address
* - We have VSIDs allocated for every kernel address
* (i.e. everything above 0xC000000000000000), except the very top
* segment, which simplifies several things.
*
* - We allow for 16 significant bits of ESID and 19 bits of
* context for user addresses. i.e. 16T (44 bits) of address space for
* up to half a million contexts.
* - We allow for USER_ESID_BITS significant bits of ESID and
* CONTEXT_BITS bits of context for user addresses.
* i.e. 64T (46 bits) of address space for up to half a million contexts.
*
* - The scramble function gives robust scattering in the hash
* - The scramble function gives robust scattering in the hash
* table (at least based on some initial results). The previous
* method was more susceptible to pathological cases giving excessive
* hash collisions.
*/
/*
* WARNING - If you change these you must make sure the asm
* implementations in slb_allocate (slb_low.S), do_stab_bolted
* (head.S) and ASM_VSID_SCRAMBLE (below) are changed accordingly.
*/
/*
* This should be computed such that protovosid * vsid_mulitplier

8
arch/powerpc/mm/mmu_context_hash64.c
View file

@ -30,9 +30,11 @@ static DEFINE_SPINLOCK(mmu_context_lock);
static DEFINE_IDA(mmu_context_ida);
/*
* The proto-VSID space has 2^35 - 1 segments available for user mappings.
* Each segment contains 2^28 bytes. Each context maps 2^44 bytes,
* so we can support 2^19-1 contexts (19 == 35 + 28 - 44).
* 256MB segment
* The proto-VSID space has 2^(CONTEX_BITS + USER_ESID_BITS) - 1 segments
* available for user mappings. Each segment contains 2^28 bytes. Each
* context maps 2^46 bytes (64TB) so we can support 2^19-1 contexts
* (19 == 37 + 28 - 46).
*/
#define MAX_CONTEXT ((1UL << CONTEXT_BITS) - 1)