Bug: PHYS_OFFSET no longer points to DRAM physical starting address with CONFIG_ARM64_VA_BITS_52

[Ard Biesheuvel]

Hello Chen-Yu,

On Sat, 22 Aug 2026, at 15:39, Chen-Yu Tsai wrote:
> Hi,
>
> On the Allwinner H6 & H616 SoC (and likely others), if running a kernel
> compiled with CONFIG_ARM64_VA_BITS_52 and CONFIG_ARM64_PA_BITS_52,
> PHYS_OFFSET no longer points to the start of DRAM (0x40000000) but instead
> points to 0xfff1000040000000.
>
>
> I've observed this for quite some time but hadn't really nailed down what
> was going on. For context: the IOMMU on these SoCs can only do 32-bit
> addresses, and that's pretty fine since the SoC only supports up to 4 GB
> DRAM. However the DRAM starts at 1GB offset in the physical address space.
> We want to be able to map the highest 1GB I/O virtual address space and
> thus need to check if the physical address is within 4GB without the
> offset. Having only 32-bits addressing, the address will wrap around fine.
> So we check the address against PHYS_OFFSE, which breaks with the mangled
> address.
>
>
> I suspect this is the result of the following commits:
>
>     7bc1a0f9e176 arm64: mm: use single quantity to represent the PA to
> VA translation
>     9684ec186f8f arm64: Enable LPA2 at boot if supported by the system
>
> The first commit introduces a negative (wrapped around) offset to
> memstart_addr (PHYS_OFFSET) to work around mapping restrictions, and the
> second commit makes the offset dependent on hardware.
>
>
> I hope that Ard and the maintainers can look into it and make PHYS_OFFSET
> always point to the physical start of DRAM.
>

On what basis are you claiming that PHYS_OFFSET must always point to the
start of physical, non-secure DRAM?

On arm64, PHYS_OFFSET is the physical address of the start of memory,
where 'start of memory' == PAGE_OFFSET, i.e., the start of the linear
map. When running a 52-bit VA kernel on hardware that is not LPA2 capable,
the hardware does not support mapping memory at PAGE_OFFSET, so it is moved
upward in the linear map.

I don't quite follow why you need to compare with PHYS_OFFSET in your IOMMU
driver: could you elaborate?

Kind regards,
-- 
Ard.