Re: [RFC PATCH 02/34] ARM: mm: make 2-level pgd_t a scalar

On Tue, Jul 14, 2026, at 15:34, David Laight wrote:
> On Tue, 14 Jul 2026 13:55:18 +0200
> "Arnd Bergmann" <[email protected]> wrote:
>> > I see the compiler has an awkward time returning a u64 struct (see
>> > https://godbolt.org/z/qejbv6j9a), but if this doesn't work maybe we should
>> > get rid of the STRICT_MM_TYPECHECKS stuff? I seriously doubt anyone is
>> > purposefully toggling it on for testing from time to time.  
>> 
>> As far as I can tell, the #ifdef was originally in i386 and
>> got copied to all other architectures at the time, but was
>> removed in linux-2.3.23 from the original copy when CONFIG_X86_PAE
>> was introduced.
>
> For x86-32 the makefiles request 64bit structures be returned in registers
> (at the same place regparm=3 is set - probably added at the same time
> between 2.4 and 2.6).
>
> Note that arm32 can will return a 32bit struct in a register and
> arm64 will return a 128bit struct in two registers.
>
> The only problem is returning a 64bit struct in 32bit mode.
>
> I'm sure this code is arm64 only.

The file we are talking about is arch/arm/include/asm/pgtable-2level-types.h,
which is definitely 32-bit only and force a 64-bit struct return
on the stack.

I checked some trivial tests that show that arm-linux-gnueabihf-gcc-16
also still produces badly optimized object code with structures
passed by value, and will spill those to the stack for no apparent
reason. clang handles those just fine as a pair of registers.

For the 2-level page table, this only concerns pgd_t, which is
rarely passed around or returned by value. The 3-level page
table has a 64-bit pte_t, which means we probably won't
want STRICT_MM_TYPECHECKS there.

      Arnd