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Date: Mon, 29 Apr 2024 18:16:54 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Sean Christopherson <seanjc@google.com>, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, 
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, "Rafael J. Wysocki" <rafael@kernel.org>, 
	Peter Zijlstra <peterz@infradead.org>, Adrian Hunter <adrian.hunter@intel.com>, 
	Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>, Elena Reshetova <elena.reshetova@intel.com>, 
	Jun Nakajima <jun.nakajima@intel.com>, Rick Edgecombe <rick.p.edgecombe@intel.com>, 
	Tom Lendacky <thomas.lendacky@amd.com>, Ashish Kalra <ashish.kalra@amd.com>, 
	Kai Huang <kai.huang@intel.com>, Baoquan He <bhe@redhat.com>, kexec@lists.infradead.org, 
	linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv10 05/18] x86/kexec: Keep CR4.MCE set during kexec for
 TDX guest
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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