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Subject[PATCH 6/8] riscv: Allow user to downgrade to sv39 when hw supports sv48
Date
This is made possible by using the mmu-type property of the cpu node of
the device tree.

By default, the kernel will boot with 4-level page table if the hw supports
it but it can be interesting for the user to select 3-level page table as
it is less memory consuming and faster since it requires less memory
accesses in case of a TLB miss.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
---
arch/riscv/mm/init.c | 25 +++++++++++++++++++++++--
1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index bad8da099ff6..1776eeb53d61 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -509,11 +509,32 @@ void disable_pgtable_l4(void)
* then read SATP to see if the configuration was taken into account
* meaning sv48 is supported.
*/
-asmlinkage __init void set_satp_mode(uintptr_t load_pa)
+asmlinkage __init void set_satp_mode(uintptr_t load_pa, uintptr_t dtb_pa)
{
u64 identity_satp, hw_satp;
int cpus_node;

+ /* 1/ Check if the user asked for sv39 explicitly in the device tree */
+ cpus_node = fdt_path_offset((void *)dtb_pa, "/cpus");
+ if (cpus_node >= 0) {
+ int node;
+
+ fdt_for_each_subnode(node, (void *)dtb_pa, cpus_node) {
+ const char *mmu_type = fdt_getprop((void *)dtb_pa, node,
+ "mmu-type", NULL);
+ if (!mmu_type)
+ continue;
+
+ if (!strcmp(mmu_type, "riscv,sv39")) {
+ disable_pgtable_l4();
+ return;
+ }
+
+ break;
+ }
+ }
+
+ /* 2/ Determine if the HW supports sv48: if not, fallback to sv39 */
create_pgd_mapping(early_pg_dir, load_pa, (uintptr_t)early_pud,
PGDIR_SIZE, PAGE_TABLE);
create_pud_mapping(early_pud, load_pa, (uintptr_t)early_pmd,
@@ -561,7 +582,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
load_sz = (uintptr_t)(&_end) - load_pa;

#if defined(CONFIG_64BIT) && !defined(CONFIG_MAXPHYSMEM_2GB)
- set_satp_mode(load_pa);
+ set_satp_mode(load_pa, dtb_pa);
#endif

kernel_virt_addr = KERNEL_VIRT_ADDR;
--
2.20.1
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