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

[James Morse <james.morse@arm.com>]

Hi,

On 16/11/15 12:41, Pavel Machek wrote:
>> 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.

I think its harder than it looks:
It means the MMU has to be turned off, as two different kernels may not
have used the same configuration for the MMU - and I don't think its safe
to change while the MMU is running. There are also going to be
complications with resetting the hypervisor/el2 configuration, which I need
to spend more time thinking about (and probably ask for advice!).


>>>> + * 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" :-).

Hah - sorry!
Fixed.


>>>> + * 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.

There aren't any other points under arch/arm64 that use the full expansion,
but it can't hurt to include both.



Thanks,

James