[PATCH v3 1/2] x86/amd_node: Avoid divide by zero on virtualized systems

[Jason Andryuk <jason.andryuk@amd.com>]

On a virtualized system, the number of nodes does not have a
relationship to the number of roots.  A Xen PVH dom0 can calculate
roots_per_node as 0, which crashes with a divide by zero in:

    if (count++ % roots_per_node)

The issue is seen with Xen, but it could affect other systems.

Change roots_per_node == 0 to 1 to allow the system to boot.  Print a
warning when this is performed for non-virtualized systems.

Fixes: 0a4b61d9c2e4 ("x86/amd_node: Fix AMD root device caching")
Cc: stable@vger.kernel.org
Suggested-by: Borislav Petkov <bp@alien8.de>
Reviewed-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
---
v3:
Add newline to warning.
R-b: Yazen

v2:
Print as a warning instead of error
Use "!" instead of "== 0".

X86_FEATURE_XENPV is only for PV, but this is observed with a PVH dom0.
---
 arch/x86/kernel/amd_node.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kernel/amd_node.c b/arch/x86/kernel/amd_node.c
index 0be01725a2a4..408b9fd48349 100644
--- a/arch/x86/kernel/amd_node.c
+++ b/arch/x86/kernel/amd_node.c
@@ -287,6 +287,11 @@ static int __init amd_smn_init(void)
 		return -ENOMEM;
 
 	roots_per_node = num_roots / num_nodes;
+	if (!roots_per_node) {
+		if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
+			pr_warn(FW_BUG "Error detecting roots per node.\n");
+		roots_per_node = 1;
+	}
 
 	count = 0;
 	node = 0;
-- 
2.55.0