[platform][qemu-virt] read the memory size out of the FDT using libfdt

-Bump the kernel forward by 64K
-Look for the FDT that qemu leaves around in the space just before the kernel
-Pick out the 'memory' node, parse the base/length values and patch the main
    pmm arena before adding it

platform/qemu-virt/platform.c

@@ -36,13 +36,14 @@
 #include <platform/gic.h>
 #include <platform/interrupts.h>
 #include <platform/qemu-virt.h>
+#include <libfdt.h>
 #include "platform_p.h"
 
 #if WITH_LIB_MINIP
 #include <lib/minip.h>
 #endif
 
-#define MEMORY_SIZE (MEMSIZE) // XXX get this from the emulator somehow
+#define DEFAULT_MEMORY_SIZE (MEMSIZE) /* try to fetch from the emulator via the fdt */
 
 /* initial memory mappings. parsed by start.S */
 struct mmu_initial_mapping mmu_initial_mappings[] = {
@@ -71,7 +72,7 @@ struct mmu_initial_mapping mmu_initial_mappings[] = {
 static pmm_arena_t arena = {
     .name = "ram",
     .base = MEMORY_BASE_PHYS,
-    .size = MEMORY_SIZE,
+    .size = DEFAULT_MEMORY_SIZE,
     .flags = PMM_ARENA_FLAG_KMAP,
 };
 
@@ -84,8 +85,53 @@ void platform_early_init(void)
 
     uart_init_early();
 
+    /* look for a flattened device tree just before the kernel */
+    const void *fdt = (void *)KERNEL_BASE;
+    int err = fdt_check_header(fdt);
+    if (err >= 0) {
+        /* walk the nodes, looking for 'memory' */
+        int depth = 0;
+        int offset = 0;
+        for (;;) {
+            offset = fdt_next_node(fdt, offset, &depth);
+            if (offset < 0)
+                break;
+
+            /* get the name */
+            const char *name = fdt_get_name(fdt, offset, NULL);
+            if (!name)
+                continue;
+
+            /* look for the 'memory' property */
+            if (strcmp(name, "memory") == 0) {
+                int lenp;
+                const void *prop_ptr = fdt_getprop(fdt, offset, "reg", &lenp);
+                if (prop_ptr && lenp == 0x10) {
+                    /* we're looking at a memory descriptor */
+                    //uint64_t base = fdt64_to_cpu(*(uint64_t *)prop_ptr);
+                    uint64_t len = fdt64_to_cpu(*((const uint64_t *)prop_ptr + 1));
+
+                    /* trim size on certain platforms */
+#if ARCH_ARM
+                    if (len > 1024*1024*1024U) {
+                        len = 1024*1024*1024; /* only use the first 1GB on ARM32 */
+                        printf("trimming memory to 1GB\n");
+                    }
+#endif
+
+                    /* set the size in the pmm arena */
+                    arena.size = len;
+                }
+            }
+        }
+    }
+
     /* add the main memory arena */
     pmm_add_arena(&arena);
+
+    /* reserve the first 64k of ram, which should be holding the fdt */
+    struct list_node list = LIST_INITIAL_VALUE(list);
+    pmm_alloc_range(MEMBASE, 0x10000 / PAGE_SIZE, &list);
 }
 
 void platform_init(void)

platform/qemu-virt/rules.mk

@@ -23,9 +23,11 @@ MODULE_SRCS += \
 
 MEMBASE := 0x40000000
 MEMSIZE ?= 0x08000000   # 512MB
+KERNEL_LOAD_OFFSET := 0x10000 # 64k
 
 MODULE_DEPS += \
     lib/cbuf \
+    lib/fdt \
     dev/interrupt/arm_gic \
     dev/timer/arm_generic \
     dev/virtio/block \

scripts/do-qemuarm

@@ -7,6 +7,7 @@ function HELP {
     echo "-t a virtio tap network device"
     echo "-d a virtio display"
     echo "-6 64bit arm"
+    echo "-m <memory in MB>"
     echo "-h for help"
     echo "all arguments after -- are passed to qemu directly"
     exit 1
@@ -17,15 +18,18 @@ DO_NET_TAP=0
 DO_BLOCK=0
 DO_64BIT=0
 DO_DISPLAY=0
+MEMSIZE=512
 SUDO=""
 
-while getopts bdhnt6 FLAG; do
+while getopts bdhm:nt6 FLAG; do
     case $FLAG in
         b) DO_BLOCK=1;;
         d) DO_DISPLAY=1;;
         n) DO_NET=1;;
         t) DO_NET_TAP=1;;
         6) DO_64BIT=1;;
+        6) DO_MEM=1;;
+        m) MEMSIZE=$OPTARG;;
         h) HELP;;
         \?)
             echo unrecognized option
@@ -43,7 +47,7 @@ else
     PROJECT="qemu-virt-a15-test"
 fi
 
-ARGS=" -m 512 -kernel build-${PROJECT}/lk.elf"
+ARGS=" -m $MEMSIZE -kernel build-${PROJECT}/lk.elf"
 BLOCK_ARGS=" -drive if=none,file=blk.bin,id=blk,format=raw -device virtio-blk-device,drive=blk"
 NET_ARGS=" -netdev user,id=vmnic,hostname=qemu -device virtio-net-device,netdev=vmnic"
 NET_TAP_ARGS=" -netdev tap,id=vmnic -device virtio-net-device,netdev=vmnic"