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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?