Re: [PATCH 2/2] ARM: LPAE: reduce damage caused by idmap to virtual memory layout

From: Konstantin Khlebnikov
Date: Mon Jul 28 2014 - 15:34:19 EST

On Mon, Jul 28, 2014 at 11:29 PM, Konstantin Khlebnikov
<koct9i@xxxxxxxxx> wrote:
> On Mon, Jul 28, 2014 at 11:13 PM, Russell King - ARM Linux
> <linux@xxxxxxxxxxxxxxxx> wrote:
>> On Mon, Jul 28, 2014 at 10:57:00PM +0400, Konstantin Khlebnikov wrote:
>>> Also I seen comment somewhere in the code which tells that idrmap pgd is
>>> always below 4gb which isn't quite true.
>> "idmap" means "identity map". It's a region which maps the same value of
>> physical address to the same value of virtual address.
>> Since the virtual address space is limited to 4GB, there is /no way/ that
>> the physical address can be above 4GB, and it still be called an
>> "identity map".
>> The reason for this is that code in the identity map will be fetched with
>> the MMU off. While this code is running, it will enable the MMU using the
>> identity map page table pointer, and the CPU must see _no_ change in the
>> instructions/data fetched from that region. It will then branch to the
>> kernel proper, and the kernel will then switch away from the identity page
>> table.
>> Once the kernel has switched away from the identity page table, interrupts
>> and other exceptions can then be taken on the CPU - but not before.
>> Neither is it expected that the CPU will access any devices until it has
>> switched away from the identity page table.
>> What this also means is that the code in the identity map must remain
>> visible in the low 4GB of physical address space.
> Ok, before switching from identity mapping to normal mapping kernel must
> switch instruction pointer from physical address to virtual. It's a long jump
> out idmap code section to some normal kernel function.
> __secondary_switched in my case. And both physical source and virtual
> destination addresses must present in one same mapping (idmap).
> idmap pgd starts as copy of pgd from init_mm, it points to the same pmd pages.
> When it populates identical mapping for that special code section it allocates
> new pages from pmd entries (which covers 1Gb of vm layout) but only few of
> them are filled. In case 3/1GB split If idmap touches somehing above 3Gb it
> kills whole kernel vm layout and as result kernel cannot jump to any
> virtual address.

Oh, my bad. That was response to Will Deacon's

> Ok. I think we need more specifics to progress with this patch. It's not
> clear to me what's going wrong or why your platform is causing the issues
> you're seeing.

>>> Moreover, I had some experiments with
>>> mapping ram to random places in qemu and seen that kernel cannot boot if
>>> PHYS_OFFSET isn't alligned to 128mb which is strange.
>> That is intentional. PHYS_OFFSET has a number of restrictions, one of
>> them is indeed that the physical offset /will/ be aligned to 128MB.
>> That was decided after looking at the platforms we have and is now
>> fixed at that value with platform-breaking consequences if it needs
>> changing.
>> --
>> FTTC broadband for 0.8mile line: currently at 9.5Mbps down 400kbps up
>> according to
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