[PATCH 6/8] ARM: mm: LPAE: Correct virt_to_phys patching for 64 bit physical addresses

[Nicolas Pitre]

On Fri, 21 Jun 2013, Santosh Shilimkar wrote:

> From: Sricharan R <r.sricharan at ti.com>
> 
> The current phys_to_virt patching mechanism does not work
> for 64 bit physical addressesp. Note that constant used in add/sub
> instructions is encoded in to the last 8 bits of the opcode. So shift
> the _pv_offset constant by 24 to get it in to the correct place.
> 
> The v2p patching mechanism patches the higher 32bits of physical
> address with a constant. While this is correct, in those platforms
> where the lowmem addressable physical memory spawns across 4GB boundary,
> a carry bit can be produced as a result of addition of lower 32bits.
> This has to be taken in to account and added in to the upper. The patched
> __pv_offset and va are added in lower 32bits, where __pv_offset can be
> in two's complement form when PA_START < VA_START and that can result
> in a false carry bit.
> 
> e.g PA = 0x80000000 VA = 0xC0000000
> __pv_offset = PA - VA = 0xC0000000 (2's complement)
> 
> So adding __pv_offset + VA should never result in a true overflow. So in
> order to differentiate between a true carry, a extra flag __pv_sign_flag
> is introduced.

I'm still wondering if this is worth bothering about.

If PA = 0x80000000 and VA = 0xC0000000 there will never be a real carry 
to propagate to the high word of the physical address as the VA space 
cannot be larger than 0x40000000.

So is there really a case where:

1) physical memory is crossing the 4GB mark, and ...

2) physical memory start address is higher than virtual memory start 
   address needing a carry due to the 32-bit add overflow?

It is easy to create (2) just by having a different user:kernel address 
space split.  However I wonder if (1) is likely.  Sure you need a memory 
alias in physical space to be able to boot, however you shouldn't need 
to address that memory alias via virtual addresses for any 
significant amount of time.  In fact, as soon as the MMU is turned on, 
there shouldn't be any issue simply using the final physical memory 
addresses right away.

What am I missing?


Nicolas