linux-kernel - Re: [PATCHv11.1 11/19] x86/tdx: Convert shared memory back to private on kexec

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Date: Tue, 4 Jun 2024 18:32:44 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Borislav Petkov <bp@...en8.de>
Cc: adrian.hunter@...el.com, ardb@...nel.org, ashish.kalra@....com, 
	bhe@...hat.com, dave.hansen@...ux.intel.com, elena.reshetova@...el.com, 
	haiyangz@...rosoft.com, hpa@...or.com, jun.nakajima@...el.com, kai.huang@...el.com, 
	kexec@...ts.infradead.org, kys@...rosoft.com, linux-acpi@...r.kernel.org, 
	linux-coco@...ts.linux.dev, linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org, 
	ltao@...hat.com, mingo@...hat.com, peterz@...radead.org, rafael@...nel.org, 
	rick.p.edgecombe@...el.com, sathyanarayanan.kuppuswamy@...ux.intel.com, seanjc@...gle.com, 
	tglx@...utronix.de, thomas.lendacky@....com, x86@...nel.org
Subject: Re: [PATCHv11.1 11/19] x86/tdx: Convert shared memory back to
 private on kexec

On Mon, Jun 03, 2024 at 10:37:54AM +0200, Borislav Petkov wrote:
> On Sun, Jun 02, 2024 at 05:23:03PM +0300, Kirill A. Shutemov wrote:
> > +			/*
> > +			 * The only thing one can do at this point on failure
> > +			 * is panic. It is reasonable to proceed.
> 
> It makes even less sense now: panic() means "all stops and we die" and
> you say it is reasonable to proceed.
> 
> I'm confused.

Right.

What about the comment below?

			/*
			 * One possible reason for the failure is if kexec raced
			 * with memory conversion. In this case shared bit in
			 * page table got set (or not cleared) during
			 * shared<->private conversion, but the page is actually
			 * private. So this failure is not going to affect the
			 * kexec'ed kernel.
			 *
			 * The only thing one can do at this point on failure
			 * at this point is panic. In absence of better options,
			 * it is reasonable to proceed, hoping the failure is a
			 * benign shared bit mismatch due to the race.
			 *
			 * Also, even if the failure is real and the page cannot
			 * be touched as private, the kdump kernel will boot
			 * fine as it uses pre-reserved memory. What happens
			 * next depends on what the dumping process does and
			 * there's a reasonable chance to produce useful dump
			 * on crash.
			 *
			 * Regardless, the print leaves a trace in the log to
			 * give a clue for debug.
			 */

-- 
  Kiryl Shutsemau / Kirill A. Shutemov