[PATCH] KVM: arm/arm64: vgic: Prevent VGIC_ADDR_TO_INTID from emiting divisions

[Marc Zyngier]

Using non-constant number of bits for VGIC_ADDR_TO_INTID() leads
to gcc 6.1 emiting calls to __aeabi_uldivmod, which the kernel
does not implement.

As we really don't want to implement complex division in the kernel,
the only other option is to prove to the compiler that there is only
a few values that are possible for the number of bits per IRQ, and
that they are all power of 2.

We turn the VGIC_ADDR_TO_INTID macro into a switch that looks for
the supported set of values (1, 2, 8, 64), and perform the computation
accordingly. When "bits" is a constant, the compiler optimizes
away the other cases. If not, we end-up with a small number of cases
that GCC optimises reasonably well. Out of range values are detected
both at build time (constants) and at run time (variables).

Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
---
This should be applied *before* Andre's patch fixing out of bound SPIs.

 virt/kvm/arm/vgic/vgic-mmio.h | 33 ++++++++++++++++++++++++++++++++-
 1 file changed, 32 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/arm/vgic/vgic-mmio.h b/virt/kvm/arm/vgic/vgic-mmio.h
index 4c34d39..a457282 100644
--- a/virt/kvm/arm/vgic/vgic-mmio.h
+++ b/virt/kvm/arm/vgic/vgic-mmio.h
@@ -57,10 +57,41 @@ extern struct kvm_io_device_ops kvm_io_gic_ops;
  * multiplication with the inverted fraction, and scale up both the
  * numerator and denominator with 8 to support at most 64 bits per IRQ:
  */
-#define VGIC_ADDR_TO_INTID(addr, bits)  (((addr) & VGIC_ADDR_IRQ_MASK(bits)) * \
+#define __VGIC_ADDR_INTID(addr, bits)  (((addr) & VGIC_ADDR_IRQ_MASK(bits)) * \
 					64 / (bits) / 8)
 
 /*
+ * Perform the same computation, but also handle non-constant number
+ * of bits. We only care about the few cases that are required by
+ * GICv2/v3.
+ */
+#define VGIC_ADDR_TO_INTID(addr, bits)				\
+	({							\
+		u32 __v;					\
+		switch((bits)) {				\
+		case 1:						\
+			__v = __VGIC_ADDR_INTID((addr), 1);	\
+			break;					\
+		case 2:						\
+			__v = __VGIC_ADDR_INTID((addr), 2);	\
+			break;					\
+		case 8:						\
+			__v = __VGIC_ADDR_INTID((addr), 8);	\
+			break;					\
+		case 64:					\
+			__v = __VGIC_ADDR_INTID((addr), 64);	\
+			break;					\
+		default:					\
+			if (__builtin_constant_p((bits)))	\
+				BUILD_BUG();			\
+			else					\
+				BUG();				\
+		}						\
+								\
+		__v;						\
+	})
+
+/*
  * Some VGIC registers store per-IRQ information, with a different number
  * of bits per IRQ. For those registers this macro is used.
  * The _WITH_LENGTH version instantiates registers with a fixed length
-- 
2.9.3