Re: [PATCH v4 13/21] arm64: entry: sdei: Make 'tsk' available

For regular entry/exit sequences, we use 'tsk' (x28) to hold the current
task pointer, allowing this to be used by various assembly macros. The
SDEI entry/exit sequence uses x28 to hold the value of the interrupted
sp_el0, and doesn't retain the current task pointer in a register.

These differences makes it awkward to share assembly macros across
regular entry/exit and SDEI entry/exit, and risk surprises.

Align the SDEI entry/exit sequence with regular entry/exit, keeping the
current task pointer in 'tsk', and preserving the interrupted sp_el0 in
another (callee-saved) register. I've used x20 as x19 was already in
used for the relevant sdei_registered_event, and these are the
lowest-numbered registers above x18 (which we must preserve for shadow
call stack).

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Muhammad Usama Anjum <redacted>
Cc: Ada Couprie Diaz <redacted>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Jinjie Ruan <redacted>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Vladimir Murzin <redacted>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <redacted>
---
 arch/arm64/kernel/entry.S | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index a6ec5c5a2d29c..0902c1bd9dd3d 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S

@@ -1023,11 +1023,12 @@ SYM_CODE_START(__sdei_asm_handler)
 
 	/*
 	 * We may have interrupted userspace, or a guest, or exit-from or
-	 * return-to either of these. We can't trust sp_el0, restore it.
+	 * return-to either of these. Preserve the interrupted sp_el0, and
+	 * initialize sp_el0 to the current task.
 	 */
-	mrs	x28, sp_el0
-	ldr_this_cpu	dst=x0, sym=__entry_task, tmp=x1
-	msr	sp_el0, x0
+	mrs	x20, sp_el0
+	ldr_this_cpu    dst=tsk, sym=__entry_task, tmp=x1
+	msr	sp_el0, tsk
 
 	/* If we interrupted the kernel point to the previous stack/frame. */
 	and     x0, x3, #0xc

@@ -1043,7 +1044,7 @@ SYM_CODE_START(__sdei_asm_handler)
 	mov	x1, x19
 	bl	__sdei_handler
 
-	msr	sp_el0, x28
+	msr	sp_el0, x20
 	/* restore regs >x17 that firmware won't restore */
 	mov	x4, x19         // keep x4 for __sdei_asm_exit_trampoline
 	ldp	x18, x19, [x4, #SDEI_EVENT_INTREGS + 16 * 9]
-- 2.30.2