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

[David Laight <david.laight.linux@gmail.com>]

On Tue, 11 Aug 2026 17:23:47 -0400
Jason Andryuk <jason.andryuk@amd.com> wrote:

> On 2026-08-11 04:21, David Laight wrote:
> > On Mon, 10 Aug 2026 10:47:06 -0400
> > Yazen Ghannam <yazen.ghannam@amd.com> 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@vger.kernel.org
> >>> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>  
> >>
> >> 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? 

No, C&P typo in the email client.

David

> The intent was to only warn if real hardware doesn't match the expectation.
> 
> Really, I just want to get this fixed.
> 
> Thanks,
> Jason