[PATCH 2/2] arm64: entry: simplify kernel_exit logic

Mark Rutland
2024-01-16T11:03:19Z

For historical reasons, the non-KPTI exception return path is duplicated for
EL1 and EL0, with the structure:

.if \el == 0
[ KPTI handling ]
ldr lr, [sp, #S_LR]
add sp, sp, #PT_REGS_SIZE // restore sp
[ EL0 exception return workaround ]
eret
.else
ldr lr, [sp, #S_LR]
add sp, sp, #PT_REGS_SIZE // restore sp
[ EL1 exception return workaround ]
eret
.endif
sb

This would be simpler and clearer with the common portions factored out,
e.g.

.if \el == 0
[ KPTI handling ]
.endif

ldr lr, [sp, #S_LR]
add sp, sp, #PT_REGS_SIZE // restore sp

.if \el == 0
[ EL0 exception return workaround ]
.else
[ EL1 exception return workaround ]
.endif

eret
sb

This expands to the same code, but is simpler for a human to follow as
it avoids duplicates the restore of LR+SP, and makes it clear that the
ERET is associated with the SB.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Will Deacon <will@kernel.org>
---
arch/arm64/kernel/entry.S | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 7fcbee0f6c0e4..7ef0e127b149f 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -442,24 +442,23 @@ alternative_else_nop_endif

.L_skip_tramp_exit_\@:
#endif
+ .endif
+
ldr lr, [sp, #S_LR]
add sp, sp, #PT_REGS_SIZE // restore sp

+ .if \el == 0
/* This must be after the last explicit memory access */
alternative_if ARM64_WORKAROUND_SPECULATIVE_UNPRIV_LOAD
tlbi vale1, xzr
dsb nsh
alternative_else_nop_endif
- eret
.else
- ldr lr, [sp, #S_LR]
- add sp, sp, #PT_REGS_SIZE // restore sp
-
/* Ensure any device/NC reads complete */
alternative_insn nop, "dmb sy", ARM64_WORKAROUND_1508412
+ .endif

eret
- .endif
sb
.endm

--
2.30.2