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Message-ID: <20240410194311.596282919@linutronix.de>
From: Thomas Gleixner <tglx@linutronix.de>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Laura Nao <laura.nao@collabora.com>, x86@kernel.org,
	kernel@collabora.com, regressions@leemhuis.info,
	"regressions@lists.linux.dev" <87il0p2era.ffs@tglx.smtp.subspace.kernel.org>
Subject: [patch 2/2] x86/cpu/amd: Make the NODEID_MSR union actually work
References: <20240410161141.261818-1-laura.nao@collabora.com>
 <87il0p2era.ffs@tglx>
 <20240410194002.909839385@linutronix.de>
Precedence: bulk
MIME-Version: 1.0
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Date: Wed, 10 Apr 2024 21:45:28 +0200 (CEST)

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))