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

Yazen Ghannam
2024-12-06T16:12:41Z

From: Mario Limonciello <mario.limonciello@amd.com>

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@amd.com>
Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
---

Notes:
Link:
https://lore.kernel.org/20241023172150.659002-17-yazen.ghannam@amd.com

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@amd.com>
*/

+#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