linux-kernel - Re: [PATCHv10 05/18] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

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Message-ID: <yh7wbdce5o35l4nkrkml3jp7tyzvzxwnfji2vmvnvaczsl6gdr@6qnwpw7g76a3>
Date: Mon, 29 Apr 2024 18:16:54 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Sean Christopherson <seanjc@...gle.com>, 
	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>, Ashish Kalra <ashish.kalra@....com>, 
	Kai Huang <kai.huang@...el.com>, Baoquan He <bhe@...hat.com>, kexec@...ts.infradead.org, 
	linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv10 05/18] x86/kexec: Keep CR4.MCE set during kexec for
 TDX guest

On Mon, Apr 29, 2024 at 04:45:08PM +0200, Borislav Petkov wrote:
> On Mon, Apr 29, 2024 at 04:17:38PM +0300, Kirill A. Shutemov wrote:
> > As I mentioned above, clearing CR4.MCE triggers #VE. It is quirk of the
> > platform.
> 
> You mean when identity_mapped() runs as part of a kexec-ed kernel, it
> might clear CR4.MCE and that would trigger the #VE?

Yes, that's what happens in current upstream.

> So, if that is correct, you basically want to *preserve* the CR4.MCE
> setting across kexec?

Yes.

> But then __mcheck_cpu_init_generic() will go and set it
> unconditionally.

__mcheck_cpu_init_generic() will not change anything in this case as the
bit is already set. Everything is hunky-dory.

> So what exactly is the correct flow here?

TDX guest has CR4.MCE set from time 0 and it has to stay this way all the
time including kexec flow.

We have already modified early boot code to preserve CR4.MCE. See
77a512e35db7 ("x86/boot: Avoid #VE during boot for TDX platforms").
The patch extends it to kexec flow.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov