Re: [PATCH 06/12] arm64: assembler: Remove endianness helper macros

[Ard Biesheuvel]

On Thu, 20 Aug 2026, at 16:19, Will Deacon wrote:
> On Sun, Aug 16, 2026 at 10:42:40AM +0100, Will Deacon wrote:
>> On Tue, Aug 11, 2026 at 05:04:43PM +0200, Ard Biesheuvel wrote:
>> > On Tue, 11 Aug 2026, at 16:01, Will Deacon wrote:
>> > > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
>> > > index 87a822e5c4ca..8951ce693552 100644
>> > > --- a/arch/arm64/kernel/head.S
>> > > +++ b/arch/arm64/kernel/head.S
>> > > @@ -138,8 +138,7 @@ SYM_CODE_START_LOCAL(record_mmu_state)
>> > >  	b.ne	0f
>> > >  	mrs	x19, sctlr_el2
>> > >  0:
>> > > -CPU_LE( tbnz	x19, #SCTLR_ELx_EE_SHIFT, 1f	)
>> > > -CPU_BE( tbz	x19, #SCTLR_ELx_EE_SHIFT, 1f	)
>> > > +	tbnz	x19, #SCTLR_ELx_EE_SHIFT, 1f
>> > >  	tst	x19, #SCTLR_ELx_C		// Z := (C == 0)
>> > >  	and	x19, x19, #SCTLR_ELx_M		// isolate M bit
>> > >  	csel	x19, xzr, x19, eq		// clear x19 if Z
>> > 
>> > There is some more code that can be removed here - see
>> > 2ced0f30a426c7301350681f838344d5aea711e3
>> 
>> Good spot, thanks! I'll do some more surgery at -rc1.
>
> Looking at this again, I'm not sure we can remove much here. I think we
> probably still want to force little-endian (i.e. clear the EE bit) if
> we're entered as big-endian. I've changed the following EOR to a BIC
> (see below), but I think that's about all we can do?
>

Well, the only case where we allow an active ID map is when doing EFI
boot, which is guaranteed to be little-endian. Since the kernel is now
also guaranteed to be little-endian, we should be able to simply kick
the CPU in LE mode right at the start, no? And simply ignore the case
of a BE bootloader entering with the MMU and caches enabled?