[PATCH 2/5] KVM: arm64: pkvm: Simplify BTI handling on CPU boot

[Marc Zyngier <maz@kernel.org>]

In order to perform an indirect branch to kvm_host_psci_cpu_entry()
on a BTI-aware system, we first branch to a 'BTI j' landing pad,
and from there branch again to the target.

While this works, this is really not required:

- BLR works with 'BTI c' and 'PACIASP' as the landing pad

- Even if LR gets clobbered by BLR, we are going to restore the
  host's registers, so it is pointless to try and avoid touching
  LR

Given the above, drop the veneer and directly call into C code.
If we were to come back from it, we'd directly enter the error
handler.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kvm/hyp/nvhe/host.S     | 10 ----------
 arch/arm64/kvm/hyp/nvhe/hyp-init.S |  9 +++++----
 2 files changed, 5 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/host.S b/arch/arm64/kvm/hyp/nvhe/host.S
index eef15b374abb0..465f6f1dd6414 100644
--- a/arch/arm64/kvm/hyp/nvhe/host.S
+++ b/arch/arm64/kvm/hyp/nvhe/host.S
@@ -291,13 +291,3 @@ SYM_CODE_START(__kvm_hyp_host_forward_smc)
 
 	ret
 SYM_CODE_END(__kvm_hyp_host_forward_smc)
-
-/*
- * kvm_host_psci_cpu_entry is called through br instruction, which requires
- * bti j instruction as compilers (gcc and llvm) doesn't insert bti j for external
- * functions, but bti c instead.
- */
-SYM_CODE_START(kvm_host_psci_cpu_entry)
-       bti j
-       b __kvm_host_psci_cpu_entry
-SYM_CODE_END(kvm_host_psci_cpu_entry)
diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-init.S b/arch/arm64/kvm/hyp/nvhe/hyp-init.S
index 5d00bde092010..55e0dce65dc56 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-init.S
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-init.S
@@ -213,12 +213,13 @@ SYM_CODE_START_LOCAL(__kvm_hyp_init_cpu)
 	mov	x0, x28
 	bl	___kvm_hyp_init			// Clobbers x0..x2
 
-	/* Leave idmap. */
+	/* Leave idmap -- using BLR is OK, LR is restored from host context */
 	mov	x0, x29
-	ldr	x1, =kvm_host_psci_cpu_entry
-	br	x1
+	ldr	x1, =__kvm_host_psci_cpu_entry
+	blr	x1
 
-	// The core booted in EL1. KVM cannot be initialized on it.
+	// The core booted in EL1, or the C code unexpectedly returned.
+	// Either way, KVM cannot be initialized on it.
 1:	wfe
 	wfi
 	b	1b
-- 
2.47.3