Re: [PATCH v2 1/2] x86/amd_node: Avoid divide by zero on virtualized systems

[Yazen Ghannam <yazen.ghannam@amd.com>]

On Mon, Aug 24, 2026 at 04:05:40PM -0400, Jason Andryuk wrote:
> Hi,
> 
> On 2026-08-14 17:42, Jason Andryuk wrote:
> > 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>
> > Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
> 
> Any feedback here?
> 
> Thanks,
> Jason
> 
> > ---
> > 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..c3e214925d9c 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.");
> > +		roots_per_node = 1;
> > +	}

Minor nit: add a newline after the closing brace.

> >   	count = 0;
> >   	node = 0;
> 

Overall, looks good to me.

Reviewed-by: Yazen Ghannam <yazen.ghannam@amd.com>

Thanks,
Yazen