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 19:14:00 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Borislav Petkov <bp@...en8.de>, 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 Tue, Jun 04, 2024 at 08:47:22AM -0700, Dave Hansen wrote:
> On 6/4/24 08:32, Kirill A. Shutemov wrote:
> > 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.
> > 			 */
> 
> It's rambling too much for my taste.
> 
> Let's boil this down to what matters:
> 
>  1. Failures to change encryption status here can lead a future kernel
>     to touch shared memory with a private mapping
>  2. That causes an immediate unrecoverable guest shutdown (right?)

Right.

>  3. kdump kernels should not be affected since they have their own
>     memory ranges and its encryption status is not being tweawked here
>  4. The pr_err() may help make some sense out of #2 when it happens
> 
> I'm not sure the reason behind the failed conversion is important here.

The important part is that failure can be benign. It explains "can" in #1.
But okay.

> I wouldn't mention panic().
> 
> We don't need to opine about what the next kernel might or might not do.

Is this any better?

			/*
			 * If tdx_enc_status_changed() fails, it leaves memory
			 * in an unknown state. If the memory remains shared,
			 * it can result in an unrecoverable guest shutdown on
			 * the first accessed through a private mapping.
			 *
			 * The kdump kernel boot is not impacted as it uses
			 * a pre-reserved memory range that is always private.
			 * However, gathering crash information could lead to
			 * a crash if it accesses unconverted memory through
			 * a private mapping.
			 *
			 * pr_err() may assist in understanding such crashes.
			 */
-- 
  Kiryl Shutsemau / Kirill A. Shutemov