[patch 2/2] x86/cpu/amd: Make the NODEID_MSR union actually work

Thomas Gleixner
2024-04-10T19:45:31Z

A system with NODEID_MSR was reported to crash during early boot without
any output.

The reason is that the union which is used for accessing the bitfields in
the MSR is written wrongly and the resulting executable code accesses the
wrong part of the MSR data.

As a consequence a later division by that value results in 0 and that
result is used for another division as divisor, which obviously does not
work well.

The magic world of C, unions and bitfields:

union {
u64 bita : 3,
bitb : 3;
u64 all;
} x;

x.all = foo();

a = x.bita;
b = x.bitb;

results in the effective executable code of:

a = b = x.bita;

because bita and bitb are treated as union members and therefore both end
up at bit offset 0.

Wrapping the bitfield into an anonymous struct:

union {
struct {
u64 bita : 3,
bitb : 3;
};
u64 all;
} x;

works like expected.

Rework the NODEID_MSR union in exactly that way to cure the problem.

Fixes: f7fb3b2dd92c ("x86/cpu: Provide an AMD/HYGON specific topology parser")
Reported-by: Laura Nao <laura.nao@collabora.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
arch/x86/kernel/cpu/topology_amd.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

--- a/arch/x86/kernel/cpu/topology_amd.c
+++ b/arch/x86/kernel/cpu/topology_amd.c
@@ -121,13 +121,13 @@ static bool parse_8000_001e(struct topo_

static bool parse_fam10h_node_id(struct topo_scan *tscan)
{
- struct {
- union {
+ union {
+ struct {
u64 node_id : 3,
nodes_per_pkg : 3,
unused : 58;
- u64 msr;
};
+ u64 msr;
} nid;

if (!boot_cpu_has(X86_FEATURE_NODEID_MSR))