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

On Mon, 24 Sep 2012, Russell King - ARM Linux wrote:

On Mon, Sep 24, 2012 at 06:32:22PM -0400, Nicolas Pitre wrote:

quoted

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

That's exactly what I mean when I say a real 64-bit addition.  Despite 
one of the arguments being a 32 bit value, the overflow needs to be 
carried to the high part.

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.)

Exactly.

And the P->V translation is truncating anyway, so that is just:

	sub	%lo, %lo, #offset

and nothing more.

Yes, that's what the posted code does already.


Nicolas