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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morse <james.morse@arm.com>
Subject: [PATCH 4.19 28/57] arm64: entry: Free up another register on kptis tramp_exit path
Date:   Mon, 21 Mar 2022 14:52:09 +0100
Message-Id: <20220321133222.807964635@linuxfoundation.org>
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From: James Morse <james.morse@arm.com>

commit 03aff3a77a58b5b52a77e00537a42090ad57b80b upstream.

Kpti stashes x30 in far_el1 while it uses x30 for all its work.

Making the vectors a per-cpu data structure will require a second
register.

Allow tramp_exit two registers before it unmaps the kernel, by
leaving x30 on the stack, and stashing x29 in far_el1.

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/arm64/kernel/entry.S |   18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -348,18 +348,20 @@ alternative_else_nop_endif
 	ldp	x24, x25, [sp, #16 * 12]
 	ldp	x26, x27, [sp, #16 * 13]
 	ldp	x28, x29, [sp, #16 * 14]
-	ldr	lr, [sp, #S_LR]
-	add	sp, sp, #S_FRAME_SIZE		// restore sp
 	/*
 	 * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on eret context synchronization
 	 * when returning from IPI handler, and when returning to user-space.
 	 */
 
 	.if	\el == 0
-alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0
+alternative_if_not ARM64_UNMAP_KERNEL_AT_EL0
+	ldr	lr, [sp, #S_LR]
+	add	sp, sp, #S_FRAME_SIZE		// restore sp
+	eret
+alternative_else_nop_endif
 #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
 	bne	4f
-	msr	far_el1, x30
+	msr	far_el1, x29
 	tramp_alias	x30, tramp_exit_native
 	br	x30
 4:
@@ -367,6 +369,8 @@ alternative_insn eret, nop, ARM64_UNMAP_
 	br	x30
 #endif
 	.else
+	ldr	lr, [sp, #S_LR]
+	add	sp, sp, #S_FRAME_SIZE		// restore sp
 	eret
 	.endif
 	.endm
@@ -996,10 +1000,12 @@ alternative_insn isb, nop, ARM64_WORKARO
 	.macro tramp_exit, regsize = 64
 	adr	x30, tramp_vectors
 	msr	vbar_el1, x30
-	tramp_unmap_kernel	x30
+	ldr	lr, [sp, #S_LR]
+	tramp_unmap_kernel	x29
 	.if	\regsize == 64
-	mrs	x30, far_el1
+	mrs	x29, far_el1
 	.endif
+	add	sp, sp, #S_FRAME_SIZE		// restore sp
 	eret
 	.endm