KVM: arm64: pkvm: Simplify BTI handling on CPU boot

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.

Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260321212419.2803972-3-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>

diff --git a/arch/arm64/kvm/hyp/nvhe/host.S b/arch/arm64/kvm/hyp/nvhe/host.S
index eef15b374abb0d0f5744d8693e6ffdba5e4a5e55..465f6f1dd6414897517c8691821637dcf5f083f3 100644 (file)
--- 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 5d00bde092010edf30589c43872bb963fbe2a583..55e0dce65dc56734405311ab66a66497000160e1 100644 (file)
--- 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