Re: [PATCH v2 11/11] arm64: kernel: Add support for hibernate/suspend-to-disk.

[Pavel Machek <pavel@ucw.cz>]

Hi!

> On 14/11/15 21:34, Pavel Machek wrote:
> >> The implementation assumes that exactly the same kernel is booted on the
> >> same hardware, and that the kernel is loaded at the same physical address.
> > 
> > BTW... on newer implementations (and I have patch for x86, too), we
> > try to make it so that resume kernel does not have to be same as
> > suspend one. It would be nice to move there with arm64, too. 
> 
> Yes, that is a neat trick, can I leave it as future work?

Yes. But it is really not hard.

> >> + * Because this code has to be copied to a safe_page, it can't call out to
> >> + * other functions by pc-relative address. Also remember that it
> > 
> > PC-relative?
> 
> The linker may (often!) use program-counter relative addresses for loads
> and stores. This code gets copied, so the linker doesn't know where the
> code will be executed from, so any instructions using pc-relative addresses
> will get the wrong result, (if they reference something outside the
> function).

I was wondering if it should be spelled "PC-relative", not
"pc-relative" :-).

> >> + * and executable pages mapped to user space are also written as data, we
> >> + * clean all pages we touch to the PoU.
> > 
> > What is PoC and PoU?
> 
> They are points in the CPU's cache hierarchy:
> 
> ARM processors are of a 'modified Harvard' architecture, their paths to
> read instructions and data are different. The 'Point of Unification' is the
> first point in the cache hierarchy that is the same for both. On ARM,
> flush_icache_range() makes sure code written as data is pushed through any
> data caches to this point, and then evicts any stale copies in the
> instruction caches.
> 
> PoC is the 'Point of Coherency', it is the first point that is the same for
> all devices, (e.g. a cpu with caches turned on, and one with them off), it
> is normally main memory. The kernel text has to be pushed to this point, so
> that secondary cores, while running early-boot code with their MMU and
> caches turned off, don't get incorrect code/data from before resume.
> 
> I have resisted the urge to draw some ascii-art!

That's ok, you just might want to replace PoI -> 'Point of
Unification' and PoC -> 'Point of Coherency' in the comments. That
should make googling easier for people not familiar with arm
terminology.

Thanks,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html