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

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

On 2026-11-04 10:18, Yazen Ghannam wrote:
> 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@vger.kernel.org
>> Suggested-by: Borislav Petkov <bp@alien8.de>
>> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
>> ---
>> 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@amd.com/
> 
> Is this available in dom0?

lspci shows 00:14.0 FCH SMBus Controller
Kernel driver in use: piix4_smbus
Kernel modules: i2c_piix4, sp5100_tco

so, yes, it seems so.

>> ---
>>   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@amd.com>
Thanks,
Jason