[PATCH v2 1/4] arm64: mm: use single quantity to represent the PA to VA translation

[Steve Capper]

Hi Ard,

On 08/10/2020 16:35, Ard Biesheuvel wrote:
> On arm64, the global variable memstart_addr represents the physical 
> address of PAGE_OFFSET, and so physical to virtual translations or 
> vice versa used to come down to simple additions or subtractions 
> involving the values of PAGE_OFFSET and memstart_addr.
> 
> When support for 52-bit virtual addressing was introduced, we had to 
> deal with PAGE_OFFSET potentially being outside of the region that can 
> be covered by the virtual range (as the 52-bit VA capable build needs 
> to be able to run on systems that are only 48-bit VA capable), and for 
> this reason, another translation was introduced, and recorded in the 
> global variable physvirt_offset.
> 
> However, if we go back to the original definition of memstart_addr, 
> i.e., the physical address of PAGE_OFFSET, it turns out that there is 
> no need for two separate translations: instead, we can simply subtract 
> the size of the unaddressable VA space from memstart_addr to make the 
> available physical memory appear in the 48-bit addressable VA region.
> 
> This simplifies things, but also fixes a bug on KASLR builds, which 
> may update memstart_addr later on in arm64_memblock_init(), but fails 
> to update vmemmap and physvirt_offset accordingly.
> 

Apologies, I didn't spot that before.

> Fixes: 5383cc6efed13 ("arm64: mm: Introduce vabits_actual")
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>

Thanks for this. It is much better to modify memstart_addr rather than introducing needless complexity.

Reviewed-by: Steve Capper <steve.capper at arm.com>

Cheers,
--
Steve