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

[Jason Andryuk <jason.andryuk@xxxxxxx>]

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@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?
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@xxxxxxx>
Thanks,
Jason