Re: [PATCH] x86/CPU/AMD: avoid printing reset reasons on Xen domU

[Sean Christopherson <seanjc@xxxxxxxxxx>]

On Fri, Dec 19, 2025, Borislav Petkov wrote:
> On December 19, 2025 1:01:31 AM UTC, Ariadne Conill <ariadne@ariadne.space> 
> wrote:
> >Xen domU cannot access the given MMIO address for security reasons,
> >resulting in a failed hypercall in ioremap() due to permissions.

Why does that matter though?  Ah, because set_pte() assumes success, and so
presumably the failed hypercall goes unnoticed and trying to access the MMIO
#PFs due to !PRESENT mapping.

> >Fixes: ab8131028710 ("x86/CPU/AMD: Print the reason for the last reset")
> >Signed-off-by: Ariadne Conill <ariadne@ariadne.space>
> >Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> >Cc: stable@xxxxxxxxxxxxxxx
> >---
> > arch/x86/kernel/cpu/amd.c | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> >diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> >index a6f88ca1a6b4..99308fba4d7d 100644
> >--- a/arch/x86/kernel/cpu/amd.c
> >+++ b/arch/x86/kernel/cpu/amd.c
> >@@ -29,6 +29,8 @@
> > # include <asm/mmconfig.h>
> > #endif
> > 
> >+#include <xen/xen.h>
> >+
> > #include "cpu.h"
> > 
> > u16 invlpgb_count_max __ro_after_init = 1;
> >@@ -1333,6 +1335,10 @@ static __init int print_s5_reset_status_mmio(void)
> >     if (!cpu_feature_enabled(X86_FEATURE_ZEN))
> >             return 0;
> > 
> >+    /* Xen PV domU cannot access hardware directly, so bail for domU case */

Heh, Xen on Zen crime.

> >+    if (cpu_feature_enabled(X86_FEATURE_XENPV) && !xen_initial_domain())
> >+            return 0;
> >+
> >     addr = ioremap(FCH_PM_BASE + FCH_PM_S5_RESET_STATUS, sizeof(value));
> >     if (!addr)
> >             return 0;
> 
> Sean, looka here. The other hypervisor wants other checks.
>
> Time to whip out the X86_FEATURE_HYPERVISOR check.

LOL, Ariadne, be honest, how much did Boris pay you?  :-D

Jokes aside, I suppose I'm fine adding a HYPERVISOR check, but at the same time,
how is this not a Xen bug?  Refusing to create a mapping because the VM doesn't
have a device defined at a given GPA is pretty hostile behavior for a 
hypervisor.