Add fwconfig command

Add a command to read values from the qemu fwcfg store. This allows data
to be passed from the qemu command line to grub.

Example use:

echo '(hd0,1)' >rootdev
qemu -fw_cfg opt/rootdev,file=rootdev

fwconfig opt/rootdev root

grub-core/Makefile.core.def

@@ -2403,3 +2403,9 @@ module = {
   common = loader/i386/xen_file64.c;
   extra_dist = loader/i386/xen_fileXX.c;
 };
+
+module = {
+  name = fwconfig;
+  common = commands/fwconfig.c;
+  enable = x86;
+};

grub-core/commands/fwconfig.c

@@ -0,0 +1,121 @@
+/* fwconfig.c - command to read config from qemu fwconfig  */
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2015  CoreOS, Inc.
+ *
+ *  GRUB is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  GRUB is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <grub/dl.h>
+#include <grub/misc.h>
+#include <grub/extcmd.h>
+#include <grub/env.h>
+#include <grub/cpu/io.h>
+#include <grub/i18n.h>
+#include <grub/mm.h>
+
+GRUB_MOD_LICENSE ("GPLv3+");
+
+#define SELECTOR 0x510
+#define DATA 0x511
+
+#define SIGNATURE_INDEX 0x00
+#define DIRECTORY_INDEX 0x19
+
+static grub_extcmd_t cmd_read_fwconfig;
+
+struct grub_qemu_fwcfgfile {
+  grub_uint32_t size;
+  grub_uint16_t select;
+  grub_uint16_t reserved;
+  char name[56];
+};
+
+static const struct grub_arg_option options[] =
+  {
+    {0, 'v', 0, N_("Save read value into variable VARNAME."),
+     N_("VARNAME"), ARG_TYPE_STRING},
+    {0, 0, 0, 0, 0, 0}
+  };
+
+static grub_err_t
+grub_cmd_fwconfig (grub_extcmd_context_t ctxt, int argc, char **argv)
+{
+  grub_uint32_t i, j, value = 0;
+  struct grub_qemu_fwcfgfile file;
+  char fwsig[4], signature[4] = { 'Q', 'E', 'M', 'U' };
+
+  if (argc != 2)
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("two arguments expected"));
+
+  /* Verify that we have meaningful hardware here */
+  grub_outw(SIGNATURE_INDEX, SELECTOR);
+  for (i=0; i<sizeof(fwsig); i++)
+      fwsig[i] = grub_inb(DATA);
+
+  if (grub_memcmp(fwsig, signature, sizeof(signature)) != 0)
+    return grub_error (GRUB_ERR_BAD_DEVICE, N_("invalid fwconfig hardware signature: got 0x%x%x%x%x"), fwsig[0], fwsig[1], fwsig[2], fwsig[3]);
+
+  /* Find out how many file entries we have */
+  grub_outw(DIRECTORY_INDEX, SELECTOR);
+  value = grub_inb(DATA) | grub_inb(DATA) << 8 | grub_inb(DATA) << 16 | grub_inb(DATA) << 24;
+  value = grub_be_to_cpu32(value);
+  /* Read the file description for each file */
+  for (i=0; i<value; i++)
+    {
+      for (j=0; j<sizeof(file); j++)
+	{
+	  ((char *)&file)[j] = grub_inb(DATA);
+	}
+      /* Check whether it matches what we're looking for, and if so read the file */
+      if (grub_strncmp(file.name, argv[0], sizeof(file.name)) == 0)
+	{
+	  grub_uint32_t filesize = grub_be_to_cpu32(file.size);
+	  grub_uint16_t location = grub_be_to_cpu16(file.select);
+	  char *data = grub_malloc(filesize+1);
+
+	  if (!data)
+	      return grub_error (GRUB_ERR_OUT_OF_MEMORY, N_("can't allocate buffer for data"));
+
+	  grub_outw(location, SELECTOR);
+	  for (j=0; j<filesize; j++)
+	    {
+	      data[j] = grub_inb(DATA);
+	    }
+
+	  data[filesize] = '\0';
+
+	  grub_env_set (argv[1], data);
+
+	  grub_free(data);
+	  return 0;
+	}
+    }
+
+  return grub_error (GRUB_ERR_FILE_NOT_FOUND, N_("couldn't find entry %s"), argv[0]);
+}
+
+GRUB_MOD_INIT(fwconfig)
+{
+  cmd_read_fwconfig =
+    grub_register_extcmd ("fwconfig", grub_cmd_fwconfig, 0,
+			  N_("PATH VAR"),
+			  N_("Set VAR to the contents of fwconfig PATH"),
+			  options);
+}
+
+GRUB_MOD_FINI(fwconfig)
+{
+  grub_unregister_extcmd (cmd_read_fwconfig);
+}