Re: [PATCH] x86/CPU/AMD: Don't print reset reason in VMs

[Yazen Ghannam <yazen.ghannam@xxxxxxx>]

On Tue, Nov 03, 2026 at 08:30:25PM -0500, Jason Andryuk wrote:
> print_s5_reset_status_mmio() expects to read FCH MMIO on baremetal.  VMs
> don't have emulated FCH registers, so there is nothing to check.  Add an
> early exit when running on a hypervisor.
> 
> The real motivation is Xen PV domUs can't boot on Zen CPUs.  PV domUs
> PTE contain system physical addresses - they don't have concept of guest
> physical addresses.  domUs can only map physical addresses they are
> assigned, so a domU fails to map the FCH MMIO addresses.  However
> set_pte() does not return failure, so ioremap() returns a pointer while
> the PTEs are unpopulated.  print_s5_reset_status_mmio() faults when
> reading through the pointer, breaking the boot.  This change fixes Xen
> PV domUs booting on Zen CPUs.
> 
> dom0 can map the FCH MMIO, but it has X86_FEATURE_HYPERVISOR set.  With
> this change, it no longer prints the reset reason.
> 
> Fixes: ab8131028710 ("x86/CPU/AMD: Print the reason for the last reset")
> Cc: stable@xxxxxxxxxxxxxxx
> Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
> Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
> ---
> dom0 could work, but that would require a xen_initial_domain() check.
> 
> I've wondered about a different approach:  We can't directly check for a
> device for the FCH_PM_BASE memory, but maybe we could check for a PCI
> ISA Bridge with PCI_VENDOR_ID_AMD.  That might be close enough to
> indicate real hardware?  But it would still be a heuristic, so I'm not
> sure how much better that would be.

Maybe, I had an older version that used the smbus device / i2c driver.
https://lore.kernel.org/all/20230407203720.18184-1-yazen.ghannam@xxxxxxx/

Is this available in dom0?

> ---
>  arch/x86/kernel/cpu/amd.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index 40463bad1078..580ccc2ce5fc 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -1357,7 +1357,8 @@ static __init int print_s5_reset_status_mmio(void)
>       u32 value;
>       int i;
>  
> -     if (!cpu_feature_enabled(X86_FEATURE_ZEN))
> +     if (!cpu_feature_enabled(X86_FEATURE_ZEN) ||
> +         cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
>               return 0;
>  
>       addr = ioremap(FCH_PM_BASE + FCH_PM_S5_RESET_STATUS, sizeof(value));
> -- 

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

Thanks,
Yazen