Re: [PATCH] x86/CPU/AMD: Ignore invalid reset reason value

[Mario Limonciello]

On 7/24/2025 3:58 PM, Sean Christopherson wrote:
> On Wed, Jul 23, 2025, Borislav Petkov wrote:
> > On July 23, 2025 9:34:26 PM GMT+03:00, Yazen Ghannam <yazen.ghannam@xxxxxxx> wrote:
> > > On Tue, Jul 22, 2025 at 06:56:15PM +0200, Borislav Petkov wrote:
> > > > On Mon, Jul 21, 2025 at 06:11:54PM +0000, Yazen Ghannam wrote:
> > > > > The reset reason value may be "all bits set", e.g. 0xFFFFFFFF. This is a
> > > > > commonly used error response from hardware. This may occur due to a real
> > > > > hardware issue or when running in a VM.
> > > >
> > > > Well, which is it Libing is reporting? VM or a real hw issue?
> > >
> > > In this case, it was a VM.
> > >
> > > > If it is a VM, is that -1 the only thing a VMM returns when reading that
> > > > MMIO address or can it be anything?
> > > >
> > > > If latter, you need to check X86_FEATURE_HYPERVISOR.
> > > >
> > > > Same for a real hw issue.
> > > >
> > > > IOW, is -1 the *only* invalid data we can read here or are we playing
> > > > whack-a-mole with it?
> > >
> > > I see you're point, but I don't think we can know for sure all possible
> > > cases. There are some reserved bits that shouldn't be set. But these
> > > definitions could change in the future.
> > >
> > > And it'd be a pain to try and verify combinations of bits and configs.
> > > Like can bit A and B be set together, or can bit C be set while running
> > > in a VM, or can bit D ever be set on Model Z?
> > >
> > > The -1 (all bits set) is the only "applies to all cases" invalid data,
> > > since this is a common hardware error response. So we can at least check
> > > for this.
> > >
> > > Thanks,
> > > Yazen
> >
> > I think you should check both: HV or -1.
> >
> > HV covers the VM angle as they don't emulate this
>
> You can't possibly know that.  If there exists a hardware spec of any kind, it's
> fair game for emulation.
>
> > and we simply should disable this functionality when running as a guest.
> >
> > -1 covers the known-bad hw value.
>
> And in a guest, -1, i.e. 0xffffffff is all but guaranteed to come from the VMM
> providing PCI master abort semantics for reads to MMIO where no device exists.
> That's about as "architectural" of behavior as you're going to get, so I don't
> see any reason to assume no VMM will every emulate whatever this feature is.

I don't really understand why there would be any value in a VMM 
emulating this feature.  It's specifically about the reason the hardware 
saw for the last reboot.  Those reasons are *hardware reasons*.  IE, 
you're never going to see a thermal event as the reason a guest was 
rebooted.

CF9 reset or ACPI power state transition are about all I can envision 
for guest reboot reasons.  And even then do you *want* the to really 
have the VMM track the reasons for a guest reboot?