[PATCH -V8 11/11] arch/powerpc: Update VSID allocation documentation
Aneesh Kumar K.V
aneesh.kumar at linux.vnet.ibm.com
Fri Sep 7 01:29:11 EST 2012
From: "Aneesh Kumar K.V" <aneesh.kumar at linux.vnet.ibm.com>
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 at samba.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com>
arch/powerpc/include/asm/mmu-hash64.h | 40 ++++++++++++++-------------------
arch/powerpc/mm/mmu_context_hash64.c | 8 ++++---
2 files changed, 22 insertions(+), 26 deletions(-)
diff --git a/arch/powerpc/include/asm/mmu-hash64.h b/arch/powerpc/include/asm/mmu-hash64.h
index de9cfed..428f23e 100644
@@ -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
diff --git a/arch/powerpc/mm/mmu_context_hash64.c b/arch/powerpc/mm/mmu_context_hash64.c
index daa076c..40bc5b0 100644
@@ -30,9 +30,11 @@ static DEFINE_SPINLOCK(mmu_context_lock);
- * 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)
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