KVM: arm64: Simplify __kvm_hyp_init HVC detection

The arguments for __do_hyp_init are now passed with a pointer to a
struct which means there are scratch registers available for use. Thanks
to this, we no longer need to use clever, but hard to read, tricks that
avoid the need for scratch registers when checking for the
__kvm_hyp_init HVC.
Tested-by: David Brazdil <dbrazdil@google.com>
Signed-off-by: Andrew Scull <ascull@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210125145415.122439-2-ascull@google.com

arch/arm64/kvm/hyp/nvhe/hyp-init.S

@@ -55,17 +55,10 @@ __do_hyp_init:
 	cmp	x0, #HVC_STUB_HCALL_NR
 	b.lo	__kvm_handle_stub_hvc
 
-	// We only actively check bits [24:31], and everything
-	// else has to be zero, which we check at build time.
-#if (KVM_HOST_SMCCC_FUNC(__kvm_hyp_init) & 0xFFFFFFFF00FFFFFF)
-#error Unexpected __KVM_HOST_SMCCC_FUNC___kvm_hyp_init value
-#endif
-
-	ror	x0, x0, #24
-	eor	x0, x0, #((KVM_HOST_SMCCC_FUNC(__kvm_hyp_init) >> 24) & 0xF)
-	ror	x0, x0, #4
-	eor	x0, x0, #((KVM_HOST_SMCCC_FUNC(__kvm_hyp_init) >> 28) & 0xF)
-	cbz	x0, 1f
+	mov	x3, #KVM_HOST_SMCCC_FUNC(__kvm_hyp_init)
+	cmp	x0, x3
+	b.eq	1f
+
 	mov	x0, #SMCCC_RET_NOT_SUPPORTED
 	eret