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Date: Wed, 8 May 2024 16:30:17 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: Borislav Petkov <bp@alien8.de>
Cc: 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>, "Kalra, Ashish" <ashish.kalra@amd.com>, 
	Sean Christopherson <seanjc@google.com>, "Huang, Kai" <kai.huang@intel.com>, Baoquan He <bhe@redhat.com>, 
	kexec@lists.infradead.org, linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org, 
	Tao Liu <ltao@redhat.com>
Subject: Re: [PATCHv10 10/18] x86/tdx: Convert shared memory back to private
 on kexec
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On Wed, May 08, 2024 at 02:04:22PM +0200, Borislav Petkov wrote:
> On Mon, May 06, 2024 at 06:37:19PM +0300, Kirill A. Shutemov wrote:
> > "second kernel" is nomenclature kexec folks are using, but okay.
> 
> And the "third kernel" is the one which got kexec-ed the second time?
> 
> You can make it: "The second, kexec-ed kernel" and then it is perfectly
> clear.

Okay.

> > One possible reason for the failure is if kdump raced with memory
> > conversion. In this case shared bit in page table got set (or not cleared
> > form shared->private conversion), but the page is actually private. So
> > this failure is not going to affect the kexec'ed kernel.
> 
> Lemme make sure I understand what you're saying here:
> 
> 1. This is a fatal failure and we should panic
> 
> However,
> 
> 2. the kexec-ed kernel is using a different page table so there won't be
>    a mismatch between shared/private marking of the page so it doesn't
>    matter
> 
> Close?

Yes.

One other point is even if the failure is real and we cannot touch the
page as private, kdump kernel will boot fine as it uses pre-reserved
memory. What happens next depends on what dumping process does. We have
reasonable chance to produce useful dump on crash.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov