[PATCH 5.10 26/58] arm64: entry: Free up another register on kptis tramp_exit path

Greg Kroah-Hartman
2022-03-10T14:34:47Z

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 | 19 +++++++++++++------
1 file changed, 13 insertions(+), 6 deletions(-)

--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -357,14 +357,16 @@ 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

.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:
@@ -372,6 +374,9 @@ alternative_insn eret, nop, ARM64_UNMAP_
br x30
#endif
.else
+ ldr lr, [sp, #S_LR]
+ add sp, sp, #S_FRAME_SIZE // restore sp
+
/* Ensure any device/NC reads complete */
alternative_insn nop, "dmb sy", ARM64_WORKAROUND_1508412

@@ -844,10 +849,12 @@ alternative_else_nop_endif
.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
sb
.endm