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

[Nicolas Pitre]

On Mon, 24 Sep 2012, Cyril Chemparathy wrote:

> 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?

I don't think that would be wise.  Let's keep only one patching pass 
with only one implementation.

It is true that the C version is more complex, mainly due to the fact 
that the instruction fixup code is more comprehensive.  However, it is 
certainly easier to maintain in the long run, and much easier to debug.

The approach where a suboptimal stub is used until the actual patching 
takes place is pretty nice.  That helps keeping the boot code small and 
simple.


Nicolas