linux-kernel - Re: [PATCHv10 04/18] cpu/hotplug, x86/acpi: Disable CPU offlining for ACPI MADT wakeup

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Date: Wed, 24 Apr 2024 11:38:42 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Adrian Hunter <adrian.hunter@...el.com>, 
	Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>, Elena Reshetova <elena.reshetova@...el.com>, 
	Jun Nakajima <jun.nakajima@...el.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	Tom Lendacky <thomas.lendacky@....com>, "Kalra, Ashish" <ashish.kalra@....com>, 
	Sean Christopherson <seanjc@...gle.com>, "Huang, Kai" <kai.huang@...el.com>, Baoquan He <bhe@...hat.com>, 
	kexec@...ts.infradead.org, linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	Tao Liu <ltao@...hat.com>
Subject: Re: [PATCHv10 04/18] cpu/hotplug, x86/acpi: Disable CPU offlining
 for ACPI MADT wakeup

On Tue, Apr 23, 2024 at 06:02:58PM +0200, Borislav Petkov wrote:
> > 
> > Currently CPU hotplug is prevented based on the confidential computing
> > attribute which is set for Intel TDX. But TDX is not the only possible
> > user of the wake up method.
> > 
> > Disable CPU offlining on ACPI MADT wakeup enumeration.
> 
> Something's missing in that "justification". It should explain why
> CC_ATTR_HOTPLUG_DISABLED is not needed anymore.

It was wrong from beginning. If ACPI MADT wake up method is used on the
platform, we cannot handle offline, regardless if it is TDX or not.

> And looking at patch 3, I'm still unclear as to why this change is done.
> Is it that the "ACPI MADT mailbox wakeup method" is going to be used by
> TDX guests now too so that you don't need CC_ATTR_HOTPLUG_DISABLED
> anymore?

ACPI MADT is the only wakeup method supported in TDX guests. But offline
is broken is because of ACPI MADT, not because of TDX.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov