linux-kernel - Re: [PATCH v5 02/22] cc_platform: Add new attribute to prevent ACPI CPU hotplug

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Date:   Fri, 24 Jun 2022 11:57:20 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     linux-acpi@...r.kernel.org, seanjc@...gle.com, pbonzini@...hat.com,
        len.brown@...el.com, tony.luck@...el.com,
        rafael.j.wysocki@...el.com, reinette.chatre@...el.com,
        dan.j.williams@...el.com, peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com, thomas.lendacky@....com,
        Tianyu.Lan@...rosoft.com, rdunlap@...radead.org, Jason@...c4.com,
        juri.lelli@...hat.com, mark.rutland@....com, frederic@...nel.org,
        yuehaibing@...wei.com, dongli.zhang@...cle.com
Subject: Re: [PATCH v5 02/22] cc_platform: Add new attribute to prevent ACPI
 CPU hotplug

On 6/22/22 04:15, Kai Huang wrote:
> Platforms with confidential computing technology may not support ACPI
> CPU hotplug when such technology is enabled by the BIOS.  Examples
> include Intel platforms which support Intel Trust Domain Extensions
> (TDX).
> 
> If the kernel ever receives ACPI CPU hotplug event, it is likely a BIOS
> bug.  For ACPI CPU hot-add, the kernel should speak out this is a BIOS
> bug and reject the new CPU.  For hot-removal, for simplicity just assume
> the kernel cannot continue to work normally, and BUG().

So, the kernel is now declaring ACPI CPU hotplug and TDX to be
incompatible and even BUG()'ing if we see them together.  Has anyone
told the firmware guys about this?  Is this in a spec somewhere?  When
the kernel goes boom, are the firmware folks going to cry "Kernel bug!!"?

This doesn't seem like something the kernel should be doing unilaterally.