[PATCH v3 RESEND 05/17] ARM: LPAE: support 64-bit virt_to_phys patching

[cyril@ti.com (Cyril Chemparathy)]

On 9/24/2012 6:40 PM, Russell King - ARM Linux wrote:
> On Mon, Sep 24, 2012 at 06:32:22PM -0400, Nicolas Pitre wrote:
>> We don't want to limit lowmem.  The lowmem is very precious memory and
>> we want to maximize its size.  In that case it is probably best to
>> implement a real patchable 64-bit addition rather than artificially
>> restricting the lowmem size.
>
> You don't need to.  You can solve the V->P translation like this:
>
>          movw    %hi, #0xVWXY            @ fixup
>          adds    %lo, %lo, #offset       @ fixup
>          adc     %hi, %hi, #0
>
> which is probably the simplest way to do the fixup - keep the existing
> fixup code, and add support for that movw instruction.  And that will
> work across any 4GB boundary just fine (we won't have more than 4GB of
> virtual address space on a 32-bit CPU anyway, so we only have to worry
> about one carry.)
>
> And the P->V translation is truncating anyway, so that is just:
>
> 	sub	%lo, %lo, #offset
>
> and nothing more.
>

The existing fixup code runs too early - before we get a chance to 
switch over from the boot-time sub-4G alias to the real physical address 
space.  Readability aside, part of the reasoning behind the C 
re-implementation was to defer the patch application until a later point 
in the boot process.

That said, we could patch once at boot, and then re-patch at switch 
over, I guess.  Is that what you're recommending?

-- 
Thanks
- Cyril