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x86/cpu/amd: Make the NODEID_MSR union actually work

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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: f7fb3b2dd9 ("x86/cpu: Provide an AMD/HYGON specific topology parser")
Reported-by: "kernelci.org bot" <bot@kernelci.org>
Reported-by: Laura Nao <laura.nao@collabora.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Laura Nao <laura.nao@collabora.com>
Link: https://lore.kernel.org/r/20240410194311.596282919@linutronix.de
Closes: https://lore.kernel.org/all/20240322175210.124416-1-laura.nao@collabora.com/
This commit is contained in:
Thomas Gleixner 2024-04-10 21:45:28 +02:00 committed by Ingo Molnar
parent 1b3108f689
commit c064b536a8
1 changed files with 3 additions and 3 deletions
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6
arch/x86/kernel/cpu/topology_amd.c
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@ -121,13 +121,13 @@ static bool parse_8000_001e(struct topo_scan *tscan, bool has_0xb)
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))