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

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Message-ID: <20250723194639.GA1229722@yaz-khff2.amd.com>
Date: Wed, 23 Jul 2025 15:46:39 -0400
From: Yazen Ghannam <yazen.ghannam@....com>
To: Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Libing He <libhe@...hat.com>, David Arcari <darcari@...hat.com>,
	Mario Limonciello <mario.limonciello@....com>
Subject: Re: [PATCH] x86/CPU/AMD: Ignore invalid reset reason value

On Wed, Jul 23, 2025 at 10:35:31PM +0300, Borislav Petkov wrote:
> On July 23, 2025 9:34:26 PM GMT+03:00, Yazen Ghannam <yazen.ghannam@....com> 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 and we simply should disable this functionality when running as a guest.
> 
> -1 covers the known-bad hw value.
> 

Okay, will do.

Thanks,
Yazen