[PATCH 1/1] Add a new grub module called msr that registers two commands (rdmsr and wrmsr) to be able to read and write to the model-specific registers. It is i386 specific, as the cpuid module. The name of the module and commands match the linux kernel module and intel commands to interact with it.

[JesusDF <jesusdf@gmail.com>]

---
 grub-core/Makefile.core.def   |  10 ++++
 grub-core/commands/i386/msr.c | 106 ++++++++++++++++++++++++++++++++++
 include/grub/i386/msr.h       |  67 +++++++++++++++++++++
 3 files changed, 183 insertions(+)
 create mode 100644 grub-core/commands/i386/msr.c
 create mode 100644 include/grub/i386/msr.h

diff --git a/grub-core/Makefile.core.def b/grub-core/Makefile.core.def
index e16fb06ba..836a353d0 100644
--- a/grub-core/Makefile.core.def
+++ b/grub-core/Makefile.core.def
@@ -2455,3 +2455,13 @@ module = {
   common = loader/i386/xen_file64.c;
   extra_dist = loader/i386/xen_fileXX.c;
 };
+
+
+module = {
+  name = msr;
+  common = commands/i386/msr.c;
+  enable = x86;
+  enable = i386_xen_pvh;
+  enable = i386_xen;
+  enable = x86_64_xen;
+};
diff --git a/grub-core/commands/i386/msr.c b/grub-core/commands/i386/msr.c
new file mode 100644
index 000000000..ea63adf97
--- /dev/null
+++ b/grub-core/commands/i386/msr.c
@@ -0,0 +1,106 @@
+/* msr.c - Read or write CPU model specific registers */
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2006, 2007, 2009  Free Software Foundation, Inc.
+ *  Based on gcc/gcc/config/i386/driver-i386.c
+ *
+ *  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/mm.h>
+#include <grub/env.h>
+#include <grub/command.h>
+#include <grub/extcmd.h>
+#include <grub/i386/msr.h>
+#include <grub/i18n.h>
+
+GRUB_MOD_LICENSE("GPLv3+");
+
+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_msr_read(grub_extcmd_context_t ctxt, int argc, char **argv)
+{
+    grub_uint64_t addr;
+    grub_uint64_t value;
+    char buf[sizeof("XXXXXXXX")];
+
+    if (argc != 1)
+    {
+        return grub_error(GRUB_ERR_BAD_ARGUMENT, N_("one argument expected"));
+    }
+
+    addr = grub_strtoul(argv[0], 0, 0);
+
+    value = grub_msr_read(addr);
+
+    if (ctxt->state[0].set)
+    {
+        grub_snprintf(buf, sizeof(buf), "%llx", value);
+        grub_env_set(ctxt->state[0].arg, buf);
+    }
+    else
+    {
+        grub_printf("0x%llx\n", value);
+    }
+
+    return GRUB_ERR_NONE;
+}
+
+static grub_err_t
+grub_cmd_msr_write(grub_command_t cmd, int argc, char **argv)
+{
+    grub_addr_t addr;
+    grub_uint32_t value;
+
+    if (argc != 2)
+    {
+        return grub_error(GRUB_ERR_BAD_ARGUMENT, N_("two arguments expected"));
+    }
+
+    addr = grub_strtoul(argv[0], 0, 0);
+    value = grub_strtoul(argv[1], 0, 0);
+
+    grub_msr_write(addr, value);
+
+    return GRUB_ERR_NONE;
+}
+
+static grub_extcmd_t cmd_read;
+static grub_extcmd_t cmd_write;
+
+GRUB_MOD_INIT(msr)
+{
+    cmd_read = grub_register_extcmd("rdmsr", grub_cmd_msr_read, 0,
+                                    N_("ADDR"),
+                                    N_("Read a CPU model specific register."),
+                                    options);
+
+    cmd_write = grub_register_command("wrmsr", grub_cmd_msr_write,
+                                      N_("ADDR VALUE"),
+                                      N_("Write a value to a CPU model specific register."));
+}
+
+GRUB_MOD_FINI(msr)
+{
+    grub_unregister_extcmd(cmd_read);
+    grub_unregister_command(cmd_write);
+}
diff --git a/include/grub/i386/msr.h b/include/grub/i386/msr.h
new file mode 100644
index 000000000..a14b3dcfb
--- /dev/null
+++ b/include/grub/i386/msr.h
@@ -0,0 +1,67 @@
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2019  Free Software Foundation, 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/>.
+ */
+
+#ifndef GRUB_CPU_MSR_HEADER
+#define GRUB_CPU_MSR_HEADER 1
+#endif
+
+#ifdef __x86_64__
+
+extern __inline grub_uint64_t grub_msr_read (grub_uint64_t msr_id)
+{
+    grub_uint32_t low;
+    grub_uint32_t high;
+    __asm__ __volatile__ ( "rdmsr"
+                           : "=a"(low), "=d"(high)
+                           : "c"(msr)
+                           );
+    return ((grub_uint64_t)high << 32) | low;
+}
+
+extern __inline void grub_msr_write(grub_uint64_t msr, grub_uint64_t value)
+{
+    grub_uint32_t low = value & 0xFFFFFFFF;
+    grub_uint32_t high = value >> 32;
+    __asm__ __volatile__ (
+                "wrmsr"
+                :
+                : "c"(msr), "a"(low), "d"(high)
+                );
+}
+
+#else
+
+extern __inline grub_uint64_t grub_msr_read (grub_uint64_t msr_id)
+{
+    /* We use uint64 in msr_id just to keep the same function signature */
+    grub_uint32_t low_id = msr_id & 0xFFFFFFFF;
+    grub_uint64_t msr_value;
+    __asm__ __volatile__ ( "rdmsr" : "=A" (msr_value) : "c" (low_id) );
+    return msr_value;
+}
+
+extern __inline void grub_msr_write(grub_uint64_t msr_id, grub_uint64_t msr_value)
+{
+    /* We use uint64 in msr_id just to keep the same function signature */
+    grub_uint32_t low_id = msr_id & 0xFFFFFFFF;
+    grub_uint32_t low_value = msr_value & 0xFFFFFFFF;
+    __asm__ __volatile__ ( "wrmsr" : : "c" (low_id), "A" (low_value) );
+}
+
+#endif
+
-- 
2.17.1