Re: [PATCH 06/12] arm64: assembler: Remove endianness helper macros
From: "Ard Biesheuvel" <ardb@kernel.org>
Date: 2026-08-20 13:52:27
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On Thu, 20 Aug 2026, at 16:45, Will Deacon wrote:
On Thu, Aug 20, 2026 at 04:27:06PM +0300, Ard Biesheuvel wrote:quoted
On Thu, 20 Aug 2026, at 16:19, Will Deacon wrote:quoted
On Sun, Aug 16, 2026 at 10:42:40AM +0100, Will Deacon wrote:quoted
On Tue, Aug 11, 2026 at 05:04:43PM +0200, Ard Biesheuvel wrote:quoted
On Tue, 11 Aug 2026, at 16:01, Will Deacon wrote:quoted
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 ZThere is some more code that can be removed here - see 2ced0f30a426c7301350681f838344d5aea711e3Good 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?Hmm, so why did we support this in the first place given that EFI is guaranteed to be little-endian? Or was that just because we wanted to handle the case of a big-endian kernel being loaded by EFI?
The original report is here: https://lore.kernel.org/linux-arm-kernel/Y9FZsBEu8hSVVIA8@dev-arch.thelio-3990X/ (local) So it would probably have been sufficient at the time to just kick the CPU into LE mode - I don't remember why I added the additional logic tbh.