The following commit has been merged into the x86/misc branch of tip:
Commit-ID: dc5243921be1b6a0b4259dbcec3dc95016ad8427
Gitweb: https://git.kernel.org/tip/dc5243921be1b6a0b4259dbcec3dc95016ad8427
Author: Yazen Ghannam <yazen.ghannam@xxxxxxx>
AuthorDate: Thu, 06 Jun 2024 11:12:57 -05:00
Committer: Borislav Petkov (AMD) <bp@xxxxxxxxx>
CommitterDate: Wed, 12 Jun 2024 11:38:58 +02:00
x86/amd_nb: Enhance SMN access error checking
AMD Zen-based systems use a System Management Network (SMN) that
provides access to implementation-specific registers.
SMN accesses are done indirectly through an index/data pair in PCI
config space. The accesses can fail for a variety of reasons.
Include code comments to describe some possible scenarios.
Require error checking for callers of amd_smn_read() and amd_smn_write().
This is needed because many error conditions cannot be checked by these
functions.
[ bp: Touchup comment. ]
Signed-off-by: Yazen Ghannam <yazen.ghannam@xxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Mario Limonciello <mario.limonciello@xxxxxxx>
Link: https://lore.kernel.org/r/20240606-fix-smn-bad-read-v4-4-ffde21931c3f@xxxxxxx
---
arch/x86/include/asm/amd_nb.h | 4 +--
arch/x86/kernel/amd_nb.c | 44 ++++++++++++++++++++++++++++++----
2 files changed, 41 insertions(+), 7 deletions(-)
diff --git a/arch/x86/include/asm/amd_nb.h b/arch/x86/include/asm/amd_nb.h
index 5c37944..6f3b6ae 100644
--- a/arch/x86/include/asm/amd_nb.h
+++ b/arch/x86/include/asm/amd_nb.h
@@ -21,8 +21,8 @@ extern int amd_numa_init(void);
extern int amd_get_subcaches(int);
extern int amd_set_subcaches(int, unsigned long);
-extern int amd_smn_read(u16 node, u32 address, u32 *value);
-extern int amd_smn_write(u16 node, u32 address, u32 value);
+int __must_check amd_smn_read(u16 node, u32 address, u32 *value);
+int __must_check amd_smn_write(u16 node, u32 address, u32 value);
struct amd_l3_cache {
unsigned indices;
diff --git a/arch/x86/kernel/amd_nb.c b/arch/x86/kernel/amd_nb.c
index 027a8c7..059e5c1 100644
--- a/arch/x86/kernel/amd_nb.c
+++ b/arch/x86/kernel/amd_nb.c
@@ -180,6 +180,43 @@ static struct pci_dev *next_northbridge(struct pci_dev *dev,
return dev;
}
+/*
+ * SMN accesses may fail in ways that are difficult to detect here in the called
+ * functions amd_smn_read() and amd_smn_write(). Therefore, callers must do
+ * their own checking based on what behavior they expect.
+ *
+ * For SMN reads, the returned value may be zero if the register is Read-as-Zero.
+ * Or it may be a "PCI Error Response", e.g. all 0xFFs. The "PCI Error Response"
+ * can be checked here, and a proper error code can be returned.
+ *
+ * But the Read-as-Zero response cannot be verified here. A value of 0 may be
+ * correct in some cases, so callers must check that this correct is for the
+ * register/fields they need.
+ *
+ * For SMN writes, success can be determined through a "write and read back"
+ * However, this is not robust when done here.
+ *
+ * Possible issues:
+ *
+ * 1) Bits that are "Write-1-to-Clear". In this case, the read value should
+ * *not* match the write value.
+ *
+ * 2) Bits that are "Read-as-Zero"/"Writes-Ignored". This information cannot be
+ * known here.
+ *
+ * 3) Bits that are "Reserved / Set to 1". Ditto above.
+ *
+ * Callers of amd_smn_write() should do the "write and read back" check
+ * themselves, if needed.
+ *
+ * For #1, they can see if their target bits got cleared.
+ *
+ * For #2 and #3, they can check if their target bits got set as intended.
+ *
+ * This matches what is done for RDMSR/WRMSR. As long as there's no #GP, then
+ * the operation is considered a success, and the caller does their own
+ * checking.
+ */
static int __amd_smn_rw(u16 node, u32 address, u32 *value, bool write)
{
struct pci_dev *root;
@@ -202,9 +239,6 @@ static int __amd_smn_rw(u16 node, u32 address, u32 *value, bool write)
err = (write ? pci_write_config_dword(root, 0x64, *value)
: pci_read_config_dword(root, 0x64, value));
- if (err)
- pr_warn("Error %s SMN address 0x%x.\n",
- (write ? "writing to" : "reading from"), address);
out_unlock:
mutex_unlock(&smn_mutex);
@@ -213,7 +247,7 @@ out:
return err;
}
-int amd_smn_read(u16 node, u32 address, u32 *value)
+int __must_check amd_smn_read(u16 node, u32 address, u32 *value)
{
int err = __amd_smn_rw(node, address, value, false);
@@ -226,7 +260,7 @@ int amd_smn_read(u16 node, u32 address, u32 *value)
}
EXPORT_SYMBOL_GPL(amd_smn_read);
-int amd_smn_write(u16 node, u32 address, u32 value)
+int __must_check amd_smn_write(u16 node, u32 address, u32 value)
{
return __amd_smn_rw(node, address, &value, true);
}
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