KVM: arm64: Define our own swab32() to avoid a uapi static inline

KVM uses swab32() when mediating GIC MMIO accesses if the GICV is badly
aligned, and the host and guest differ in endianness.

arm64 doesn't provide a __arch_swab32(), so __fswab32() is always backed
by the macro implementation that the compiler reduces to a single
instruction. But the static-inline causes problems for KVM if the compiler
chooses not to inline this function, it may not be located in the
__hyp_text where __vgic_v2_perform_cpuif_access() needs it.

Create our own __kvm_swab32() macro that calls ___constant_swab32()
directly. This way we know it will always be inlined.
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200220165839.256881-3-james.morse@arm.com

arch/arm64/include/asm/kvm_hyp.h

@@ -47,6 +47,13 @@
 #define read_sysreg_el2(r)	read_sysreg_elx(r, _EL2, _EL1)
 #define write_sysreg_el2(v,r)	write_sysreg_elx(v, r, _EL2, _EL1)
 
+/*
+ * Without an __arch_swab32(), we fall back to ___constant_swab32(), but the
+ * static inline can allow the compiler to out-of-line this. KVM always wants
+ * the macro version as its always inlined.
+ */
+#define __kvm_swab32(x)	___constant_swab32(x)
+
 int __vgic_v2_perform_cpuif_access(struct kvm_vcpu *vcpu);
 
 void __vgic_v3_save_state(struct kvm_vcpu *vcpu);

arch/arm64/kvm/hyp/vgic-v2-cpuif-proxy.c

@@ -69,14 +69,14 @@ int __hyp_text __vgic_v2_perform_cpuif_access(struct kvm_vcpu *vcpu)
 		u32 data = vcpu_get_reg(vcpu, rd);
 		if (__is_be(vcpu)) {
 			/* guest pre-swabbed data, undo this for writel() */
-			data = swab32(data);
+			data = __kvm_swab32(data);
 		}
 		writel_relaxed(data, addr);
 	} else {
 		u32 data = readl_relaxed(addr);
 		if (__is_be(vcpu)) {
 			/* guest expects swabbed data */
-			data = swab32(data);
+			data = __kvm_swab32(data);
 		}
 		vcpu_set_reg(vcpu, rd, data);
 	}