[PATCH v2 16/16] x86/amd_node: Add support for debugfs access to SMN registers

[Yazen Ghannam <yazen.ghannam@xxxxxxx>]

From: Mario Limonciello <mario.limonciello@xxxxxxx>

There are certain registers on AMD Zen systems that can only be
accessed through SMN.

Introduce a new interface that provides debugfs files for accessing SMN.
As this introduces the capability for userspace to manipulate the
hardware in unpredictable ways, taint the kernel when writing.

Include a kernel parameter to enable the debugfs interface. This is
intentionally left undocumented to discourage use of the interface.

Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
Signed-off-by: Yazen Ghannam <yazen.ghannam@xxxxxxx>
---

Notes:
    Link:
    https://lore.kernel.org/20241023172150.659002-17-yazen.ghannam@xxxxxxx
    
    v1->v2:
    * Use TAINT_CPU_OUT_OF_SPEC.
    * Add parameter to enable debugfs interface.
    * Validate node input from user.

 arch/x86/kernel/amd_node.c | 99 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)

diff --git a/arch/x86/kernel/amd_node.c b/arch/x86/kernel/amd_node.c
index ac571948cb35..b670fa85c61b 100644
--- a/arch/x86/kernel/amd_node.c
+++ b/arch/x86/kernel/amd_node.c
@@ -8,6 +8,7 @@
  * Author: Yazen Ghannam <Yazen.Ghannam@xxxxxxx>
  */
 
+#include <linux/debugfs.h>
 #include <asm/amd_node.h>
 
 /*
@@ -192,6 +193,87 @@ int __must_check amd_smn_hsmp_rdwr(u16 node, u32 address, u32 *value, bool write
 }
 EXPORT_SYMBOL_GPL(amd_smn_hsmp_rdwr);
 
+static struct dentry *debugfs_dir;
+static u16 debug_node;
+static u32 debug_address;
+
+static ssize_t smn_node_write(struct file *file, const char __user *userbuf,
+			      size_t count, loff_t *ppos)
+{
+	u16 node;
+	int ret;
+
+	ret = kstrtou16_from_user(userbuf, count, 0, &node);
+	if (ret)
+		return ret;
+
+	if (node >= amd_num_nodes())
+		return -ENODEV;
+
+	debug_node = node;
+	return count;
+}
+
+static int smn_node_show(struct seq_file *m, void *v)
+{
+	seq_printf(m, "0x%08x\n", debug_node);
+	return 0;
+}
+
+static ssize_t smn_address_write(struct file *file, const char __user *userbuf,
+				 size_t count, loff_t *ppos)
+{
+	int ret;
+
+	ret = kstrtouint_from_user(userbuf, count, 0, &debug_address);
+	if (ret)
+		return ret;
+
+	return count;
+}
+
+static int smn_address_show(struct seq_file *m, void *v)
+{
+	seq_printf(m, "0x%08x\n", debug_address);
+	return 0;
+}
+
+static int smn_value_show(struct seq_file *m, void *v)
+{
+	u32 val;
+	int ret;
+
+	ret = amd_smn_read(debug_node, debug_address, &val);
+	if (ret)
+		return ret;
+
+	seq_printf(m, "0x%08x\n", val);
+	return 0;
+}
+
+static ssize_t smn_value_write(struct file *file, const char __user *userbuf,
+			       size_t count, loff_t *ppos)
+{
+	u32 val;
+	int ret;
+
+	ret = kstrtouint_from_user(userbuf, count, 0, &val);
+	if (ret)
+		return ret;
+
+	add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+
+	ret = amd_smn_write(debug_node, debug_address, val);
+	if (ret)
+		return ret;
+
+	return count;
+}
+
+DEFINE_SHOW_STORE_ATTRIBUTE(smn_node);
+DEFINE_SHOW_STORE_ATTRIBUTE(smn_address);
+DEFINE_SHOW_STORE_ATTRIBUTE(smn_value);
+
 static int amd_cache_roots(void)
 {
 	u16 node, num_nodes = amd_num_nodes();
@@ -239,6 +321,15 @@ static int reserve_root_config_spaces(void)
 	return 0;
 }
 
+static bool enable_dfs;
+
+static int __init amd_smn_enable_dfs(char *str)
+{
+	enable_dfs = true;
+	return 1;
+}
+__setup("amd_smn_debugfs_enable", amd_smn_enable_dfs);
+
 static int __init amd_smn_init(void)
 {
 	int err;
@@ -259,6 +350,14 @@ static int __init amd_smn_init(void)
 	if (err)
 		return err;
 
+	if (enable_dfs) {
+		debugfs_dir = debugfs_create_dir("amd_smn", arch_debugfs_dir);
+
+		debugfs_create_file("node",	0600, debugfs_dir, NULL, &smn_node_fops);
+		debugfs_create_file("address",	0600, debugfs_dir, NULL, &smn_address_fops);
+		debugfs_create_file("value",	0600, debugfs_dir, NULL, &smn_value_fops);
+	}
+
 	return 0;
 }
 
-- 
2.43.0