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RISC-V: Setup initial page tables in two stages

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Currently, the setup_vm() does initial page table setup in one-shot
very early before enabling MMU. Due to this, the setup_vm() has to map
all possible kernel virtual addresses since it does not know size and
location of RAM. This means we have kernel mappings for non-existent
RAM and any buggy driver (or kernel) code doing out-of-bound access
to RAM will not fault and cause underterministic behaviour.

Further, the setup_vm() creates PMD mappings (i.e. 2M mappings) for
RV64 systems. This means for PAGE_OFFSET=0xffffffe000000000 (i.e.
MAXPHYSMEM_128GB=y), the setup_vm() will require 129 pages (i.e.
516 KB) of memory for initial page tables which is never freed. The
memory required for initial page tables will further increase if
we chose a lower value of PAGE_OFFSET (e.g. 0xffffff0000000000)

This patch implements two-staged initial page table setup, as follows:
1. Early (i.e. setup_vm()): This stage maps kernel image and DTB in
a early page table (i.e. early_pg_dir). The early_pg_dir will be used
only by boot HART so it can be freed as-part of init memory free-up.
2. Final (i.e. setup_vm_final()): This stage maps all possible RAM
banks in the final page table (i.e. swapper_pg_dir). The boot HART
will start using swapper_pg_dir at the end of setup_vm_final(). All
non-boot HARTs directly use the swapper_pg_dir created by boot HART.

We have following advantages with this new approach:
1. Kernel mappings for non-existent RAM don't exists anymore.
2. Memory consumed by initial page tables is now indpendent of the
chosen PAGE_OFFSET.
3. Memory consumed by initial page tables on RV64 system is 2 pages
(i.e. 8 KB) which has significantly reduced and these pages will be
freed as-part of the init memory free-up.

The patch also provides a foundation for implementing strict kernel
mappings where we protect kernel text and rodata using PTE permissions.

Suggested-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
[paul.walmsley@sifive.com: updated to apply; fixed a checkpatch warning]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
This commit is contained in:
Anup Patel
2019-06-28 13:36:21 -07:00
committed by Paul Walmsley
parent 2ebca1cbb4
commit 671f9a3e2e
6 changed files with 286 additions and 66 deletions
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17
arch/riscv/kernel/head.S
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@@ -55,7 +55,9 @@ clear_bss_done:
/* Initialize page tables and relocate to virtual addresses */
la sp, init_thread_union + THREAD_SIZE
mv a0, s1
call setup_vm
la a0, early_pg_dir
call relocate
/* Restore C environment */
@@ -64,25 +66,23 @@ clear_bss_done:
la sp, init_thread_union + THREAD_SIZE
/* Start the kernel */
mv a0, s1
call parse_dtb
tail start_kernel
relocate:
/* Relocate return address */
li a1, PAGE_OFFSET
la a0, _start
sub a1, a1, a0
la a2, _start
sub a1, a1, a2
add ra, ra, a1
/* Point stvec to virtual address of intruction after satp write */
la a0, 1f
add a0, a0, a1
csrw CSR_STVEC, a0
la a2, 1f
add a2, a2, a1
csrw CSR_STVEC, a2
/* Compute satp for kernel page tables, but don't load it yet */
la a2, swapper_pg_dir
srl a2, a2, PAGE_SHIFT
srl a2, a0, PAGE_SHIFT
li a1, SATP_MODE
or a2, a2, a1
@@ -148,6 +148,7 @@ relocate:
fence
/* Enable virtual memory and relocate to virtual address */
la a0, swapper_pg_dir
call relocate
tail smp_callin