xen/riscv: introduce function for physical offset calculation

The function was introduced to calculate and save physical
offset before MMU is enabled because access to start() is
PC-relative and in case of linker_addr != load_addr it will
result in incorrect value in phys_offset.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/riscv/include/asm/mm.h

@@ -15,4 +15,6 @@ void setup_initial_pagetables(void);
 void enable_mmu(void);
 void cont_after_mmu_is_enabled(void);
 
+unsigned long calc_phys_offset(void);
+
 #endif /* _ASM_RISCV_MM_H */

xen/arch/riscv/mm.c

@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
+#include <xen/cache.h>
 #include <xen/compiler.h>
 #include <xen/init.h>
 #include <xen/kernel.h>
@@ -19,9 +20,10 @@ struct mmu_desc {
     pte_t *pgtbl_base;
 };
 
-#define PHYS_OFFSET ((unsigned long)_start - XEN_VIRT_START)
-#define LOAD_TO_LINK(addr) ((addr) - PHYS_OFFSET)
-#define LINK_TO_LOAD(addr) ((addr) + PHYS_OFFSET)
+static unsigned long __ro_after_init phys_offset;
+
+#define LOAD_TO_LINK(addr) ((unsigned long)(addr) - phys_offset)
+#define LINK_TO_LOAD(addr) ((unsigned long)(addr) + phys_offset)
 
 /*
  * It is expected that Xen won't be more then 2 MB.
@@ -273,3 +275,16 @@ void __init noreturn noinline enable_mmu()
     switch_stack_and_jump((unsigned long)cpu0_boot_stack + STACK_SIZE,
                           cont_after_mmu_is_enabled);
 }
+
+/*
+ * calc_phys_offset() should be used before MMU is enabled because access to
+ * start() is PC-relative and in case when load_addr != linker_addr phys_offset
+ * will have an incorrect value
+ */
+unsigned long __init calc_phys_offset(void)
+{
+    volatile unsigned long load_start = (unsigned long)_start;
+
+    phys_offset = load_start - XEN_VIRT_START;
+    return phys_offset;
+}

xen/arch/riscv/riscv64/head.S

@@ -29,6 +29,20 @@ ENTRY(start)
 
         jal     reset_stack
 
+        /*
+         * save hart_id ( bootcpu_id ) and dtb_base as a0 and a1 register can
+         * be used by C code
+         */
+        mv      s0, a0
+        mv      s1, a1
+
+        jal     calc_phys_offset
+        mv      s2, a0
+
+        /* restore hart_id ( bootcpu_id ) and dtb address */
+        mv      a0, s0
+        mv      a1, s1
+
         tail    start_xen
 
         .section .text, "ax", %progbits