[PATCH 2/2] hw/riscv: sifive_u: Provide a reliable way for bootloader to detect whether it is running in QEMU

[Bin Meng <bmeng.cn@gmail.com>]

From: Bin Meng <bin.meng@windriver.com>

The reset vector codes are subject to change, e.g.: with recent
fw_dynamic type image support, it breaks oreboot again.

Add a subregion in the MROM, with the size of machine RAM stored,
so that we can provide a reliable way for bootloader to detect
whether it is running in QEMU.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
---

 hw/riscv/sifive_u.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/hw/riscv/sifive_u.c b/hw/riscv/sifive_u.c
index 3413369..6d714a2 100644
--- a/hw/riscv/sifive_u.c
+++ b/hw/riscv/sifive_u.c
@@ -88,6 +88,7 @@ static const struct MemmapEntry {
 
 #define OTP_SERIAL          1
 #define GEM_REVISION        0x10070109
+#define MROM_RAMSIZE_OFFSET 0xf8
 
 static void create_fdt(SiFiveUState *s, const struct MemmapEntry *memmap,
     uint64_t mem_size, const char *cmdline)
@@ -382,6 +383,7 @@ static void sifive_u_machine_init(MachineState *machine)
     int i;
     uint32_t fdt_load_addr;
     uint64_t kernel_entry;
+    ram_addr_t ram_size = machine->ram_size;
 
     /* Initialize SoC */
     object_initialize_child(OBJECT(machine), "soc", &s->soc, TYPE_RISCV_U_SOC);
@@ -391,7 +393,7 @@ static void sifive_u_machine_init(MachineState *machine)
 
     /* register RAM */
     memory_region_init_ram(main_mem, NULL, "riscv.sifive.u.ram",
-                           machine->ram_size, &error_fatal);
+                           ram_size, &error_fatal);
     memory_region_add_subregion(system_memory, memmap[SIFIVE_U_DRAM].base,
                                 main_mem);
 
@@ -406,7 +408,7 @@ static void sifive_u_machine_init(MachineState *machine)
                           qemu_allocate_irq(sifive_u_machine_reset, NULL, 0));
 
     /* create device tree */
-    create_fdt(s, memmap, machine->ram_size, machine->kernel_cmdline);
+    create_fdt(s, memmap, ram_size, machine->kernel_cmdline);
 
     if (s->start_in_flash) {
         /*
@@ -443,7 +445,7 @@ static void sifive_u_machine_init(MachineState *machine)
         if (machine->initrd_filename) {
             hwaddr start;
             hwaddr end = riscv_load_initrd(machine->initrd_filename,
-                                           machine->ram_size, kernel_entry,
+                                           ram_size, kernel_entry,
                                            &start);
             qemu_fdt_setprop_cell(s->fdt, "/chosen",
                                   "linux,initrd-start", start);
@@ -460,7 +462,7 @@ static void sifive_u_machine_init(MachineState *machine)
 
     /* Compute the fdt load address in dram */
     fdt_load_addr = riscv_load_fdt(memmap[SIFIVE_U_DRAM].base,
-                                   machine->ram_size, s->fdt);
+                                   ram_size, s->fdt);
     #if defined(TARGET_RISCV64)
     start_addr_hi32 = start_addr >> 32;
     #endif
@@ -496,6 +498,17 @@ static void sifive_u_machine_init(MachineState *machine)
     riscv_rom_copy_firmware_info(memmap[SIFIVE_U_MROM].base,
                                  memmap[SIFIVE_U_MROM].size,
                                  sizeof(reset_vec), kernel_entry);
+
+    /*
+     * Tell guest the machine ram size at MROM_RAMSIZE_OFFSET.
+     * On real hardware, the 64-bit value from MROM_RAMSIZE_OFFSET is zero.
+     * QEMU aware bootloader (e.g.: oreboot, U-Boot) can check value stored
+     * here to determine whether it is running in QEMU.
+     */
+    ram_size = cpu_to_le32(ram_size);
+    rom_add_blob_fixed_as("mrom.ram_size", &ram_size, sizeof(ram_size),
+                          memmap[SIFIVE_U_MROM].base + MROM_RAMSIZE_OFFSET,
+                          &address_space_memory);
 }
 
 static bool sifive_u_machine_get_start_in_flash(Object *obj, Error **errp)
-- 
2.7.4