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Subject[PATCH 03/18] arm64: kvm: stop treating register x18 as caller save
From: Ard Biesheuvel <>

In preparation of using x18 as a task struct pointer register when
running in the kernel, stop treating it as caller save in the KVM
guest entry/exit code. Currently, the code assumes there is no need
to preserve it for the host, given that it would have been assumed
clobbered anyway by the function call to __guest_enter(). Instead,
preserve its value and restore it upon return.

Signed-off-by: Ard Biesheuvel <>
Signed-off-by: Sami Tolvanen <>
arch/arm64/kvm/hyp/entry.S | 12 +++++-------
1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index e5cc8d66bf53..20bd9a20ea27 100644
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -23,6 +23,7 @@
.pushsection .hyp.text, "ax"

.macro save_callee_saved_regs ctxt
+ str x18, [\ctxt, #CPU_XREG_OFFSET(18)]
stp x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
stp x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
stp x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
@@ -38,6 +39,7 @@
ldp x25, x26, [\ctxt, #CPU_XREG_OFFSET(25)]
ldp x27, x28, [\ctxt, #CPU_XREG_OFFSET(27)]
ldp x29, lr, [\ctxt, #CPU_XREG_OFFSET(29)]
+ ldr x18, [\ctxt, #CPU_XREG_OFFSET(18)]

@@ -87,12 +89,9 @@ alternative_else_nop_endif
ldp x14, x15, [x18, #CPU_XREG_OFFSET(14)]
ldp x16, x17, [x18, #CPU_XREG_OFFSET(16)]

- // Restore guest regs x19-x29, lr
+ // Restore guest regs x18-x29, lr
restore_callee_saved_regs x18

- // Restore guest reg x18
- ldr x18, [x18, #CPU_XREG_OFFSET(18)]
// Do not touch any register after this!
@@ -114,7 +113,7 @@ ENTRY(__guest_exit)
// Retrieve the guest regs x0-x1 from the stack
ldp x2, x3, [sp], #16 // x0, x1

- // Store the guest regs x0-x1 and x4-x18
+ // Store the guest regs x0-x1 and x4-x17
stp x2, x3, [x1, #CPU_XREG_OFFSET(0)]
stp x4, x5, [x1, #CPU_XREG_OFFSET(4)]
stp x6, x7, [x1, #CPU_XREG_OFFSET(6)]
@@ -123,9 +122,8 @@ ENTRY(__guest_exit)
stp x12, x13, [x1, #CPU_XREG_OFFSET(12)]
stp x14, x15, [x1, #CPU_XREG_OFFSET(14)]
stp x16, x17, [x1, #CPU_XREG_OFFSET(16)]
- str x18, [x1, #CPU_XREG_OFFSET(18)]

- // Store the guest regs x19-x29, lr
+ // Store the guest regs x18-x29, lr
save_callee_saved_regs x1

get_host_ctxt x2, x3
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