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Update the description of the address space layout on RISC-V.

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This adds more detail and fixes some inaccuracies.

Reviewed by:	jhb
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D18463
This commit is contained in:
Mark Johnston 2018-12-07 15:56:40 +00:00
parent 1f5e341b46
commit 1a153f42fa
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=341694
1 changed files with 14 additions and 15 deletions
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29
sys/riscv/include/vmparam.h
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@ -115,22 +115,23 @@
/**
* Address space layout.
*
* RISC-V implements up to a 48 bit virtual address space. The address space is
* split into 2 regions at each end of the 64 bit address space, with an
* out of range "hole" in the middle.
* RISC-V implements multiple paging modes with different virtual address space
* sizes: SV32, SV39 and SV48. SV39 permits a virtual address space size of
* 512GB and uses a three-level page table. Since this is large enough for most
* purposes, we currently use SV39 for both userland and the kernel, avoiding
* the extra translation step required by SV48.
*
* We limit the size of the two spaces to 39 bits each.
* The address space is split into two regions at each end of the 64-bit address
* space:
*
* Upper region: 0xffffffffffffffff
* 0xffffff8000000000
* 0x0000000000000000 - 0x0000003fffffffff 256GB user map
* 0x0000004000000000 - 0xffffffbfffffffff unmappable
* 0xffffffc000000000 - 0xffffffc7ffffffff 32GB kernel map
* 0xffffffc800000000 - 0xffffffcfffffffff 32GB unused
* 0xffffffd000000000 - 0xffffffefffffffff 128GB direct map
* 0xfffffff000000000 - 0xffffffffffffffff 64GB unused
*
* Hole: 0xffffff7fffffffff
* 0x0000008000000000
*
* Lower region: 0x0000007fffffffff
* 0x0000000000000000
*
* We use the upper region for the kernel, and the lower region for userland.
* The kernel is loaded at the beginning of the kernel map.
*
* We define some interesting address constants:
*
@ -146,11 +147,9 @@
#define VM_MIN_ADDRESS (0x0000000000000000UL)
#define VM_MAX_ADDRESS (0xffffffffffffffffUL)
/* 32 GiB of kernel addresses */
#define VM_MIN_KERNEL_ADDRESS (0xffffffc000000000UL)
#define VM_MAX_KERNEL_ADDRESS (0xffffffc800000000UL)
/* 128 GiB maximum for the direct map region */
#define DMAP_MIN_ADDRESS (0xffffffd000000000UL)
#define DMAP_MAX_ADDRESS (0xfffffff000000000UL)