powerpc/mm: Update VSID allocation documentation (f033d659) · Commits · e / devices / android_kernel_fairphone_FP3

arch/powerpc/include/asm/mmu-hash64.h

+17 −23

Original line number	Diff line number	Diff line
		@@ -324,27 +324,26 @@ 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).
		@@ -355,20 +354,15 @@ extern void slb_set_size(u16 size);
		* (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
		* 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

arch/powerpc/mm/mmu_context_hash64.c

+5 −3

Original line number	Diff line number	Diff line
		@@ -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)