riscv: Use PUD/P4D/PGD pages for the linear mapping

During the early page table creation, we used to set the mapping for
PAGE_OFFSET to the kernel load address: but the kernel load address is
always offseted by PMD_SIZE which makes it impossible to use PUD/P4D/PGD
pages as this physical address is not aligned on PUD/P4D/PGD size (whereas
PAGE_OFFSET is).

But actually we don't have to establish this mapping (ie set va_pa_offset)
that early in the boot process because:

- first, setup_vm installs a temporary kernel mapping and among other
  things, discovers the system memory,
- then, setup_vm_final creates the final kernel mapping and takes
  advantage of the discovered system memory to create the linear
  mapping.

During the first phase, we don't know the start of the system memory and
then until the second phase is finished, we can't use the linear mapping at
all and phys_to_virt/virt_to_phys translations must not be used because it
would result in a different translation from the 'real' one once the final
mapping is installed.

So here we simply delay the initialization of va_pa_offset to after the
system memory discovery. But to make sure noone uses the linear mapping
before, we add some guard in the DEBUG_VIRTUAL config.

Finally we can use PUD/P4D/PGD hugepages when possible, which will result
in a better TLB utilization.

Note that:
- this does not apply to rv32 as the kernel mapping lies in the linear
  mapping.
- we rely on the firmware to protect itself using PMP.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Acked-by: Rob Herring <robh@kernel.org> # DT bits
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Tested-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20230324155421.271544-4-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

1 files changed, 52 insertions, 6 deletions

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index ed07ae111f3a..7bd66795165d 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -213,6 +213,14 @@ static void __init setup_bootmem(void)
 	phys_ram_end = memblock_end_of_DRAM();
 	if (!IS_ENABLED(CONFIG_XIP_KERNEL))
 		phys_ram_base = memblock_start_of_DRAM();
+
+	/*
+	 * In 64-bit, any use of __va/__pa before this point is wrong as we
+	 * did not know the start of DRAM before.
+	 */
+	if (IS_ENABLED(CONFIG_64BIT))
+		kernel_map.va_pa_offset = PAGE_OFFSET - phys_ram_base;
+
 	/*
 	 * memblock allocator is not aware of the fact that last 4K bytes of
 	 * the addressable memory can not be mapped because of IS_ERR_VALUE
@@ -667,9 +675,16 @@ void __init create_pgd_mapping(pgd_t *pgdp,
 
 static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size)
 {
-	/* Upgrade to PMD_SIZE mappings whenever possible */
-	base &= PMD_SIZE - 1;
-	if (!base && size >= PMD_SIZE)
+	if (!(base & (PGDIR_SIZE - 1)) && size >= PGDIR_SIZE)
+		return PGDIR_SIZE;
+
+	if (!(base & (P4D_SIZE - 1)) && size >= P4D_SIZE)
+		return P4D_SIZE;
+
+	if (!(base & (PUD_SIZE - 1)) && size >= PUD_SIZE)
+		return PUD_SIZE;
+
+	if (!(base & (PMD_SIZE - 1)) && size >= PMD_SIZE)
 		return PMD_SIZE;
 
 	return PAGE_SIZE;
@@ -978,11 +993,22 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
 	set_satp_mode();
 #endif
 
-	kernel_map.va_pa_offset = PAGE_OFFSET - kernel_map.phys_addr;
+	/*
+	 * In 64-bit, we defer the setup of va_pa_offset to setup_bootmem,
+	 * where we have the system memory layout: this allows us to align
+	 * the physical and virtual mappings and then make use of PUD/P4D/PGD
+	 * for the linear mapping. This is only possible because the kernel
+	 * mapping lies outside the linear mapping.
+	 * In 32-bit however, as the kernel resides in the linear mapping,
+	 * setup_vm_final can not change the mapping established here,
+	 * otherwise the same kernel addresses would get mapped to different
+	 * physical addresses (if the start of dram is different from the
+	 * kernel physical address start).
+	 */
+	kernel_map.va_pa_offset = IS_ENABLED(CONFIG_64BIT) ?
+				0UL : PAGE_OFFSET - kernel_map.phys_addr;
 	kernel_map.va_kernel_pa_offset = kernel_map.virt_addr - kernel_map.phys_addr;
 
-	phys_ram_base = kernel_map.phys_addr;
-
 	/*
 	 * The default maximal physical memory size is KERN_VIRT_SIZE for 32-bit
 	 * kernel, whereas for 64-bit kernel, the end of the virtual address
@@ -1106,6 +1132,17 @@ static void __init create_linear_mapping_page_table(void)
 	phys_addr_t start, end;
 	u64 i;
 
+#ifdef CONFIG_STRICT_KERNEL_RWX
+	phys_addr_t ktext_start = __pa_symbol(_start);
+	phys_addr_t ktext_size = __init_data_begin - _start;
+	phys_addr_t krodata_start = __pa_symbol(__start_rodata);
+	phys_addr_t krodata_size = _data - __start_rodata;
+
+	/* Isolate kernel text and rodata so they don't get mapped with a PUD */
+	memblock_mark_nomap(ktext_start,  ktext_size);
+	memblock_mark_nomap(krodata_start, krodata_size);
+#endif
+
 	/* Map all memory banks in the linear mapping */
 	for_each_mem_range(i, &start, &end) {
 		if (start >= end)
@@ -1118,6 +1155,15 @@ static void __init create_linear_mapping_page_table(void)
 
 		create_linear_mapping_range(start, end);
 	}
+
+#ifdef CONFIG_STRICT_KERNEL_RWX
+	create_linear_mapping_range(ktext_start, ktext_start + ktext_size);
+	create_linear_mapping_range(krodata_start,
+				    krodata_start + krodata_size);
+
+	memblock_clear_nomap(ktext_start,  ktext_size);
+	memblock_clear_nomap(krodata_start, krodata_size);
+#endif
 }
 
 static void __init setup_vm_final(void)