riscv: Support CONFIG_RELOCATABLE on NOMMU

Move relocate_kernel() out of the CONFIG_MMU block so it can be called
from the NOMMU version of setup_vm(). Set some offsets in kernel_map so
relocate_kernel() does not need to be modified. Relocatable NOMMU
kernels can be loaded to any physical memory address; they no longer
depend on CONFIG_PAGE_OFFSET.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Link: https://lore.kernel.org/r/20241026171441.3047904-4-samuel.holland@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: SiFiveHolland

arch/riscv/Kconfig

@@ -1075,7 +1075,7 @@ config PARAVIRT_TIME_ACCOUNTING
 
 config RELOCATABLE
 	bool "Build a relocatable kernel"
-	depends on MMU && 64BIT && !XIP_KERNEL
+	depends on 64BIT && !XIP_KERNEL
 	select MODULE_SECTIONS if MODULES
 	help
           This builds a kernel as a Position Independent Executable (PIE),

arch/riscv/include/asm/pgtable.h

@@ -12,7 +12,11 @@
 #include <asm/pgtable-bits.h>
 
 #ifndef CONFIG_MMU
+#ifdef CONFIG_RELOCATABLE
+#define KERNEL_LINK_ADDR	UL(0)
+#else
 #define KERNEL_LINK_ADDR	_AC(CONFIG_PAGE_OFFSET, UL)
+#endif
 #define KERN_VIRT_SIZE		(UL(-1))
 #else
 

arch/riscv/mm/init.c

@@ -323,6 +323,44 @@ static void __init setup_bootmem(void)
 		hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT);
 }
 
+#ifdef CONFIG_RELOCATABLE
+extern unsigned long __rela_dyn_start, __rela_dyn_end;
+
+static void __init relocate_kernel(void)
+{
+	Elf64_Rela *rela = (Elf64_Rela *)&__rela_dyn_start;
+	/*
+	 * This holds the offset between the linked virtual address and the
+	 * relocated virtual address.
+	 */
+	uintptr_t reloc_offset = kernel_map.virt_addr - KERNEL_LINK_ADDR;
+	/*
+	 * This holds the offset between kernel linked virtual address and
+	 * physical address.
+	 */
+	uintptr_t va_kernel_link_pa_offset = KERNEL_LINK_ADDR - kernel_map.phys_addr;
+
+	for ( ; rela < (Elf64_Rela *)&__rela_dyn_end; rela++) {
+		Elf64_Addr addr = (rela->r_offset - va_kernel_link_pa_offset);
+		Elf64_Addr relocated_addr = rela->r_addend;
+
+		if (rela->r_info != R_RISCV_RELATIVE)
+			continue;
+
+		/*
+		 * Make sure to not relocate vdso symbols like rt_sigreturn
+		 * which are linked from the address 0 in vmlinux since
+		 * vdso symbol addresses are actually used as an offset from
+		 * mm->context.vdso in VDSO_OFFSET macro.
+		 */
+		if (relocated_addr >= KERNEL_LINK_ADDR)
+			relocated_addr += reloc_offset;
+
+		*(Elf64_Addr *)addr = relocated_addr;
+	}
+}
+#endif /* CONFIG_RELOCATABLE */
+
 #ifdef CONFIG_MMU
 struct pt_alloc_ops pt_ops __meminitdata;
 
@@ -893,44 +931,6 @@ static __init void set_satp_mode(uintptr_t dtb_pa)
 #error "setup_vm() is called from head.S before relocate so it should not use absolute addressing."
 #endif
 
-#ifdef CONFIG_RELOCATABLE
-extern unsigned long __rela_dyn_start, __rela_dyn_end;
-
-static void __init relocate_kernel(void)
-{
-	Elf64_Rela *rela = (Elf64_Rela *)&__rela_dyn_start;
-	/*
-	 * This holds the offset between the linked virtual address and the
-	 * relocated virtual address.
-	 */
-	uintptr_t reloc_offset = kernel_map.virt_addr - KERNEL_LINK_ADDR;
-	/*
-	 * This holds the offset between kernel linked virtual address and
-	 * physical address.
-	 */
-	uintptr_t va_kernel_link_pa_offset = KERNEL_LINK_ADDR - kernel_map.phys_addr;
-
-	for ( ; rela < (Elf64_Rela *)&__rela_dyn_end; rela++) {
-		Elf64_Addr addr = (rela->r_offset - va_kernel_link_pa_offset);
-		Elf64_Addr relocated_addr = rela->r_addend;
-
-		if (rela->r_info != R_RISCV_RELATIVE)
-			continue;
-
-		/*
-		 * Make sure to not relocate vdso symbols like rt_sigreturn
-		 * which are linked from the address 0 in vmlinux since
-		 * vdso symbol addresses are actually used as an offset from
-		 * mm->context.vdso in VDSO_OFFSET macro.
-		 */
-		if (relocated_addr >= KERNEL_LINK_ADDR)
-			relocated_addr += reloc_offset;
-
-		*(Elf64_Addr *)addr = relocated_addr;
-	}
-}
-#endif /* CONFIG_RELOCATABLE */
-
 #ifdef CONFIG_XIP_KERNEL
 static void __init create_kernel_page_table(pgd_t *pgdir,
 					    __always_unused bool early)
@@ -1378,6 +1378,12 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
 {
 	dtb_early_va = (void *)dtb_pa;
 	dtb_early_pa = dtb_pa;
+
+#ifdef CONFIG_RELOCATABLE
+	kernel_map.virt_addr = (uintptr_t)_start;
+	kernel_map.phys_addr = (uintptr_t)_start;
+	relocate_kernel();
+#endif
 }
 
 static inline void setup_vm_final(void)