KVM: arm64: Don't corrupt tpidr_el2 on failed HVC call

The hyp-init code starts by stashing a register in TPIDR_EL2
in in order to free a register. This happens no matter if the
HVC call is legal or not.

Although nothing wrong seems to come out of it, it feels odd
to alter the EL2 state for something that eventually returns
an error.

Instead, use the fact that we know exactly which bits of the
__kvm_hyp_init call are non-zero to perform the check with
a series of EOR/ROR instructions, combined with a build-time
check that the value is the one we expect.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201026095116.72051-2-maz@kernel.org

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

@@ -57,16 +57,25 @@ __do_hyp_init:
 	cmp	x0, #HVC_STUB_HCALL_NR
 	b.lo	__kvm_handle_stub_hvc
 
-	/* Set tpidr_el2 for use by HYP to free a register */
-	msr	tpidr_el2, x2
-
-	mov	x2, #KVM_HOST_SMCCC_FUNC(__kvm_hyp_init)
-	cmp	x0, x2
-	b.eq	1f
+	// 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	x0, #SMCCC_RET_NOT_SUPPORTED
 	eret
 
-1:	phys_to_ttbr x0, x1
+1:
+	/* Set tpidr_el2 for use by HYP to free a register */
+	msr	tpidr_el2, x2
+
+	phys_to_ttbr x0, x1
 alternative_if ARM64_HAS_CNP
 	orr	x0, x0, #TTBR_CNP_BIT
 alternative_else_nop_endif