Xen PVH booting via multiboot protocol (from pvgrub2-pvh)

bindings/xen/boot.S

@@ -20,10 +20,29 @@
 
 #include "../cpu_x86_64.h"
 #include "xen/elfnote.h"
+#include "../virtio/multiboot.h"
 
 #define ENTRY(x) .text; .globl x; .type x,%function; x:
 #define END(x)   .size x, . - x
 
+#define XEN_HVM_START_MAGIC_VALUE 0x336ec578
+
+#define MYMULTIBOOT_FLAGS \
+    (MULTIBOOT_PAGE_ALIGN | MULTIBOOT_MEMORY_INFO | MULTIBOOT_AOUT_KLUDGE)
+
+.section .data.multiboot
+
+    .align 4
+    _multiboot_header:
+    .long MULTIBOOT_HEADER_MAGIC
+    .long MYMULTIBOOT_FLAGS
+    .long -(MULTIBOOT_HEADER_MAGIC+MYMULTIBOOT_FLAGS)
+    .long _multiboot_header
+    .long 0x100000
+    .long _edata
+    .long _ebss
+    .long _start32
+
 /*
  * Tell Xen that we are a PVH-capable kernel.
  * See https://xenbits.xen.org/docs/unstable/misc/pvh.html.
@@ -42,19 +61,33 @@
 /*
  * Xen PVH entry point.
  *
- * Xen puts us only in 32bit flat protected mode, and passes a pointer to
- * struct hvm_start_info in %ebx. It's our responsibility to install a page
- * table and switch to long mode.  Notably, we can't call C code until we've
- * switched to long mode.
+ * When booted directly, Xen puts us only in 32bit flat protected mode, and
+ * passes a pointer to struct hvm_start_info in %ebx. When booted via grub's
+ * multiboot protocol, grub passes a pointer to struct multiboot_info in %ebx
+ * and sets a magic in %eax. Otherwise both boot modes works the same. The
+ * platform_init() differentiate the structure based on a magic at the
+ * beginning of hvm_start_info.
+ * It's our responsibility to install a page table and switch to long mode.
+ * Notably, we can't call C code until we've switched to long mode.
  */
 ENTRY(_start32)
     cld
     movl $bootstack, %esp
 
-    /* Save Xen hvm_start_info pointer at top of stack, we pop it in 64bit */
+    /*
+     * Save Xen hvm_start_info or multiboot_info pointer (depending on boot
+     * mode) at top of stack, we pop it in 64bit
+     */
     pushl $0
     pushl %ebx
 
+    cmpl $MULTIBOOT_BOOTLOADER_MAGIC, %eax
+    je 1f
+    movl (%ebx), %eax
+    cmpl $XEN_HVM_START_MAGIC_VALUE, %eax
+    jne unknownboot
+
+1:
     /*
      * Load bootstrap GDT and reload segment registers, with the exception of
      * CS, which will be reloaded on jumping to _start64
@@ -104,6 +137,8 @@ ENTRY(_start32)
     /* NOTREACHED */
     jmp haltme
 
+unknownboot:
+
 haltme:
     cli
     hlt

bindings/xen/platform.c

@@ -24,6 +24,7 @@
 #include "xen/arch-x86/cpuid.h"
 #include "xen/arch-x86/hvm/start_info.h"
 #include "xen/hvm/params.h"
+#include "../virtio/multiboot.h"
 
 /*
  * Xen shared_info page is mapped here.
@@ -227,19 +228,64 @@ static void hypercall_init(void)
     assert(HYPERCALL_PAGE[0] != 0xc3);
 }
 
-void platform_init(const void *arg __attribute__((unused)))
+static void parse_multiboot(const void *arg)
 {
-    pagetable_init();
-    hypercall_init();
-    console_init();
+    /*
+     * The multiboot structures may be anywhere in memory, so take a copy of
+     * the command line before we initialise memory allocation.
+     */
+    const struct multiboot_info *mi = (struct multiboot_info *)arg;
+
+    if (mi->flags & MULTIBOOT_INFO_CMDLINE) {
+        char *mi_cmdline = (char *)(uint64_t)mi->cmdline;
+        size_t cmdline_len = strlen(mi_cmdline);
+
+        /*
+         * Skip the first token in the cmdline as it is an opaque "name" for
+         * the kernel coming from the bootloader.
+         */
+        for (; *mi_cmdline; mi_cmdline++, cmdline_len--) {
+            if (*mi_cmdline == ' ') {
+                mi_cmdline++;
+                cmdline_len--;
+                break;
+            }
+        }
+
+        if (cmdline_len >= sizeof(cmdline)) {
+            cmdline_len = sizeof(cmdline) - 1;
+            log(WARN, "Solo5: warning: command line too long, truncated\n");
+        }
+        memcpy(cmdline, mi_cmdline, cmdline_len);
+    } else {
+        cmdline[0] = 0;
+    }
 
+    /*
+     * Look for the first chunk of memory at PLATFORM_MEM_START.
+     */
+    multiboot_memory_map_t *m = NULL;
+    uint32_t offset;
+
+    for (offset = 0; offset < mi->mmap_length;
+            offset += m->size + sizeof(m->size)) {
+        m = (void *)(uintptr_t)(mi->mmap_addr + offset);
+        if (m->addr == PLATFORM_MEM_START &&
+                m->type == MULTIBOOT_MEMORY_AVAILABLE) {
+            break;
+        }
+    }
+    assert(offset < mi->mmap_length);
+    mem_size = m->addr + m->len;
+}
+
+static void parse_hvm_start_info(const void *arg)
+{
     /*
      * The Xen hvm_start_info may be anywhere in memory, so take a copy of
      * the command line before we initialise memory allocation.
      */
     const struct hvm_start_info *si = (struct hvm_start_info *)arg;
-    assert(si->magic == XEN_HVM_START_MAGIC_VALUE);
-
     if (si->cmdline_paddr) {
         char *hvm_cmdline = (char *)si->cmdline_paddr;
         size_t cmdline_len = strlen(hvm_cmdline);
@@ -295,6 +341,35 @@ void platform_init(const void *arg __attribute__((unused)))
             }
         }
     }
+}
+
+void platform_init(const void *arg)
+{
+    bool is_direct_pvh_boot;
+
+    if (*((uint32_t*)arg) == XEN_HVM_START_MAGIC_VALUE)
+        is_direct_pvh_boot = true;
+    else
+        is_direct_pvh_boot = false;
+
+    /*
+     * If booted via multiboot, parse multiboot structure before setting up
+     * page tables as its location may not be mapped later.
+     */
+    if (!is_direct_pvh_boot)
+        parse_multiboot(arg);
+
+    pagetable_init();
+    hypercall_init();
+    console_init();
+
+    /*
+     * If booted directly, parse the Xen hvm_start_info structure after setting
+     * up hypercalls
+     */
+    if (is_direct_pvh_boot)
+        parse_hvm_start_info(arg);
+
     assert(mem_size);
 
     /*

bindings/xen/solo5_xen.lds

@@ -55,16 +55,17 @@ SECTIONS {
      */
     _stext = .;
 
-    .interp : {
-        *(.interp)
-    } :interp :text
-
     .text :
     {
+        *(.data.multiboot)
         *(.text)
         *(.text.*)
     } :text
 
+    .interp : {
+        *(.interp)
+    } :interp :text
+
     . = ALIGN(CONSTANT(MAXPAGESIZE));
     _etext = .;