linux-kernel - Re: [PATCH v6 2/2] ARM hibernation / suspend-to-disk

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Message-ID: <20140228224933.GA11040@e102568-lin.cambridge.arm.com>
Date:	Fri, 28 Feb 2014 22:49:33 +0000
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	Sebastian Capella <sebastian.capella@...aro.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Russ Dill <Russ.Dill@...com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Russell King <linux@....linux.org.uk>,
	Len Brown <len.brown@...el.com>,
	Nicolas Pitre <nico@...aro.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Will Deacon <Will.Deacon@....com>,
	Jonathan Austin <Jonathan.Austin@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Uwe Kleine-K?nig <u.kleine-koenig@...gutronix.de>,
	Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [PATCH v6 2/2] ARM hibernation / suspend-to-disk

On Fri, Feb 28, 2014 at 08:15:57PM +0000, Sebastian Capella wrote:

[...]

> > > +
> > > +/*
> > > + * The framework loads the hibernation image into a linked list anchored
> > > + * at restore_pblist, for swsusp_arch_resume() to copy back to the proper
> > > + * destinations.
> > > + *
> > > + * To make this work if resume is triggered from initramfs, the
> > > + * pagetables need to be switched to allow writes to kernel mem.
> > > + */
> > 
> > Comment above needs updating. We are switching page tables to a set of
> > page tables that are certain to live at the same location in the older
> > kernel, that's the only reason, as we discussed. soft_restart will make
> > sure (again) to switch to 1:1 page tables so that we can call cpu_resume
> > with the MMU off.
> 
> How does this look?
> 
> The framework loads as much of the hibernation image to final physical
> pages as possible.  Any pages that were in use, will need to be restored
> prior to the soft_restart.  The pages to restore are maintained in
> the list anchored at restore_pblist.  At this point, we can swap the
> pages to their final location.  We must switch the mapping to 1:1 to
> ensure that when we overwrite the page table physical pages we're using
> a known physical location (idmap_pgd) with known contents.

It is ok, a tad too verbose. All I care about is a comment describing
what's really needed, the existing one was confusing and wrong.

> > > +/*
> > > + * Resume from the hibernation image.
> > > + * Due to the kernel heap / data restore, stack contents change underneath
> > > + * and that would make function calls impossible; switch to a temporary
> > > + * stack within the nosave region to avoid that problem.
> > > + */
> > > +int swsusp_arch_resume(void)
> > > +{
> > > +     extern void call_with_stack(void (*fn)(void *), void *arg, void *sp);
> > > +     call_with_stack(arch_restore_image, 0,
> > > +             resume_stack + sizeof(resume_stack));
> > 
> > This does not guarantee your stack is 8-byte aligned, that's not AAPCS
> > compliant and might buy you trouble.
> > 
> > Either you align the stack or you align the pointer you are passing.
> > 
> > Please have a look at kernel/process.c
> 
> I've added this for now, do you see any issues?
> 
> -static u8 resume_stack[PAGE_SIZE/2] __nosavedata;
> +static u64 resume_stack[PAGE_SIZE/2/sizeof(u64)] __nosavedata;
> -               resume_stack + sizeof(resume_stack));
> +               resume_stack + ARRAY_SIZE(resume_stack));

I do not see why the stack depends on the PAGE_SIZE. I would be surprised
if you need more than a few bytes (given that soft_restart switches stack
again...), go through it with a debugger, it is easy to check the stack
usage and allow for some extra buffer (but half a page is not needed).

My main concern was alignment, and now that's fixed.

Thanks !
Lorenzo

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