Re: [PATCH v3 6/6] arm64/mm: Drop configurable 48-bit physical address space limit

[Nick Chan]

Hi Ard,

On 12/12/2024 16:18, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> 
> Currently, the maximum supported physical address space can be
> configured as either 48 bits or 52 bits. The only remaining difference
> between these in practice is that the former omits the masking and
> shifting required to construct TTBR and PTE values, which carry bits #48
> and higher disjoint from the rest of the physical address.
> 
> The overhead of performing these additional calculations is negligible,
> and so there is little reason to retain support for two different
> configurations, and we can simply support whatever the hardware
> supports.

I am seeing a boot failure on Apple iPad 7 which uses 
CONFIG_ARM64_16K=y after this change in linux-next as commit
32d053d6f5e9, with nothing appearing on serial console with
earlycon enabled unless I set CONFIG_ARM64_VA_BITS_52=y. Reverting
this patch makes the kernel work again.

Nick Chan