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

[Jason Andryuk]

On 2026-08-11 04:21, David Laight wrote:
> On Mon, 10 Aug 2026 10:47:06 -0400
> Yazen Ghannam <yazen.ghannam@xxxxxxx> wrote:
>
> > On Thu, Aug 06, 2026 at 12:01:57PM -0400, 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)
> > >
> > > On a virtualized system, default the value to 1.  The issue is seen with
> > > Xen, but it could affect other systems.
> > >
> > > Fixes: 0a4b61d9c2e4 ("x86/amd_node: Fix AMD root device caching")
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
> >
> > I agree with the idea with some minor feedback below.
> >
> > > ---
> > > 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 ea553267e5fa..c5025e5291b6 100644
> > > --- a/arch/x86/kernel/amd_node.c
> > > +++ b/arch/x86/kernel/amd_node.c
> > > @@ -286,6 +286,11 @@ static int __init amd_smn_init(void)
> > >   		return -ENOMEM;
> > >   
> > >   	roots_per_node = num_roots / num_nodes;
> > > +	if (roots_per_node == 0) {
> >
> > Can be '!roots_per_node'.
> >
> > > +		if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
> > > +			pr_err(FW_BUG "Error detecting roots per node.\n");
> > > +		roots_per_node = 1;
> > > +	}
> >
> > I don't think this is necessarily a Firmware bug.
> >
> > Likewise, it can be a 'warning' rather than 'error'. I interpret 'error'
> > as something known to be incorrect. This was my position before on this
> > topic. But the various contrary reports changed my mind (even if they
> > come from virtualization).
> >
> > Furthermore, I think 'warning' is more appropriate. It says "We found
> > something unexpected. We're letting you (the user) know about it. And
> > we'll mitigate it to avoid an error."
> >
> > Anyways, I just wanted to write out some thoughts since this topic has
> > come up a few times.
> >
> > Possible rework for the above change:
> >
> > 	if (!roots_per_node && !cpu_feature_enabled(X86_FEATURE_HYPERVISOR)
> > 		pr_warn("Expected at least 1 root per AMD node.\n");
> >
> > 	roots_per_node = max(roots_per_node, 1);
>
> I'd not add the max(), just;
> 	if (!roots_per_node) {
> 		roots_per_node = 1;
> 		if (cpu_feature_enabled(X86_FEATURE_HYPERVISOR)
> 			pr_warn("Expected at least 1 root per AMD node.\n");
> 	}

I prefer this because it puts the special handing inside the check. 
David, you dropped ! from X86_FEATURE_HYPERVISOR - did you mean that? 
The intent was to only warn if real hardware doesn't match the expectation.

Really, I just want to get this fixed.

Thanks,
Jason