[PATCH v3 3/6] x86/amd_node: Restrict SMN root enumeration to AMD vendor

[Lin Wang]

Drop X86_VENDOR_HYGON / PCI_VENDOR_ID_HYGON support from the AMD SMN
init path:

  - get_next_root(): only match PCI_VENDOR_ID_AMD host bridges.
  - amd_smn_init(): add boot_cpu_data.x86_vendor != X86_VENDOR_AMD to
    the early-return condition, alongside the existing X86_FEATURE_ZEN
    check (X86_FEATURE_ZEN is set on Hygon Dhyana too, so the feature
    check alone is not sufficient to gate AMD-only init).

This is functionally required by the Hygon DF node and SMN access
implementation added in subsequent patches.  Before this cleanup, on
a Hygon Family 0x18 platform:

  amd_smn_init (fs_initcall)
    -> get_next_root() matched Hygon host bridges
    -> pci_request_config_region_exclusive() reserved Hygon root
       PCI config space
    -> smn_exclusive = true

That left Hygon's own SMN setup unable to reserve the same config
space, failing with -EEXIST.  Restricting both sites to AMD vendor
leaves Hygon root devices untouched by the AMD path so the
Hygon-specific SMN implementation in arch/x86/kernel/hygon_node.c
can reserve them itself.

Together, these changes keep the AMD SMN path AMD-only: root
discovery matches AMD host bridges only, and amd_smn_init() returns
early on non-AMD CPUs.

Gating amd_smn_init() on vendor also changes a precondition the SMN
access path relied on.  On Hygon, amd_smn_init() now returns early, so
amd_roots stays NULL and smn_exclusive stays false, while
amd_num_nodes() (derived from CPU topology) remains non-zero.  Harden
__amd_smn_rw() to test smn_exclusive before indexing amd_roots[], so a
caller that has not yet been converted to the Hygon SMN path receives
-ENODEV instead of dereferencing a NULL amd_roots[].  Additionally,
amd_smn_read() now returns the underlying error directly instead of
reading back a possibly-uninitialised *value, and zeroes *value on any
failure.  Neither change affects AMD SMN access after successful
initialization.

Signed-off-by: Lin Wang <wanglin@xxxxxxxxxxxxxx>
---
 arch/x86/kernel/amd_node.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/amd_node.c b/arch/x86/kernel/amd_node.c
index 0be01725a2a4..62086b8b5804 100644
--- a/arch/x86/kernel/amd_node.c
+++ b/arch/x86/kernel/amd_node.c
@@ -88,16 +88,17 @@ static int __amd_smn_rw(u8 i_off, u8 d_off, u16 node, u32 address, u32 *value, b
 	struct pci_dev *root;
 	int err = -ENODEV;
 
-	if (node >= amd_num_nodes())
+	/*
+	 * smn_exclusive is set only after amd_roots is allocated and the
+	 * root config space reserved; test it before indexing amd_roots[].
+	 */
+	if (!smn_exclusive || node >= amd_num_nodes())
 		return err;
 
 	root = amd_roots[node];
 	if (!root)
 		return err;
 
-	if (!smn_exclusive)
-		return err;
-
 	guard(mutex)(&smn_mutex);
 
 	err = pci_write_config_dword(root, i_off, address);
@@ -116,9 +117,15 @@ int __must_check amd_smn_read(u16 node, u32 address, u32 *value)
 {
 	int err = __amd_smn_rw(SMN_INDEX_OFFSET, SMN_DATA_OFFSET, node, address, value, false);
 
+	/* On error *value may be untouched; do not read it back. */
+	if (err) {
+		*value = 0;
+		return err;
+	}
+
 	if (PCI_POSSIBLE_ERROR(*value)) {
-		err = -ENODEV;
 		*value = 0;
+		return -ENODEV;
 	}
 
 	return err;
@@ -225,8 +232,7 @@ static struct pci_dev *get_next_root(struct pci_dev *root)
 		if (root->devfn)
 			continue;
 
-		if (root->vendor != PCI_VENDOR_ID_AMD &&
-		    root->vendor != PCI_VENDOR_ID_HYGON)
+		if (root->vendor != PCI_VENDOR_ID_AMD)
 			continue;
 
 		break;
@@ -249,7 +255,8 @@ static int __init amd_smn_init(void)
 	u16 count, num_roots, roots_per_node, node, num_nodes;
 	struct pci_dev *root;
 
-	if (!cpu_feature_enabled(X86_FEATURE_ZEN))
+	if (!cpu_feature_enabled(X86_FEATURE_ZEN) ||
+	    boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
 		return 0;
 
 	guard(mutex)(&smn_mutex);
-- 
2.43.0