Re: [PATCH v1 1/1] x86/fred: Fix system hang during S4 resume with FRED enabled

[Ingo Molnar <mingo@kernel.org>]

* Rafael J. Wysocki <rafael@kernel.org> wrote:

> On Wed, Mar 26, 2025 at 7:26 AM Xin Li (Intel) <xin@zytor.com> wrote:
> >
> > During an S4 resume, the system first performs a cold power-on.  The
> > kernel image is initially loaded to a random linear address, and the
> > FRED MSRs are initialized.  Subsequently, the S4 image is loaded,
> > and the kernel image is relocated to its original address from before
> > the S4 suspend.  Due to changes in the kernel text and data mappings,
> > the FRED MSRs must be reinitialized.
> 
> To be precise, the above description of the hibernation control flow
> doesn't exactly match the code.
> 
> Yes, a new kernel is booted upon a wakeup from S4, but this is not "a
> cold power-on", strictly speaking.  This kernel is often referred to
> as the restore kernel and yes, it initializes the FRED MSRs as
> appropriate from its perspective.
> 
> Yes, it loads a hibernation image, including the kernel that was
> running before hibernation, often referred to as the image kernel, but
> it does its best to load image pages directly into the page frames
> occupied by them before hibernation unless those page frames are
> currently in use.  In that case, the given image pages are loaded into
> currently free page frames, but they may or may not be part of the
> image kernel (they may as well belong to user space processes that
> were running before hibernation).  Yes, all of these pages need to be
> moved to their original locations before the last step of restore,
> which is a jump into a "trampoline" page in the image kernel, but this
> is sort of irrelevant to the issue at hand.
> 
> At this point, the image kernel has control, but the FRED MSRs still
> contain values written to them by the restore kernel and there is no
> guarantee that those values are the same as the ones written into them
> by the image kernel before hibernation.  Thus the image kernel must
> ensure that the values of the FRED MSRs will be the same as they were
> before hibernation, and because they only depend on the location of
> the kernel text and data, they may as well be recomputed from scratch.

That's a rather critical difference... I zapped the commit from 
tip:x86/urgent, awaiting -v2 with a better changelog and better
in-code comments.

Thanks,

	Ingo