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-rw-r--r--virt/kvm/arm/vgic/vgic-mmio.h14
1 files changed, 7 insertions, 7 deletions
diff --git a/virt/kvm/arm/vgic/vgic-mmio.h b/virt/kvm/arm/vgic/vgic-mmio.h
index 4c34d39d44a0..84961b4e4422 100644
--- a/virt/kvm/arm/vgic/vgic-mmio.h
+++ b/virt/kvm/arm/vgic/vgic-mmio.h
@@ -50,15 +50,15 @@ extern struct kvm_io_device_ops kvm_io_gic_ops;
#define VGIC_ADDR_IRQ_MASK(bits) (((bits) * 1024 / 8) - 1)
/*
- * (addr & mask) gives us the byte offset for the INT ID, so we want to
- * divide this with 'bytes per irq' to get the INT ID, which is given
- * by '(bits) / 8'. But we do this with fixed-point-arithmetic and
- * take advantage of the fact that division by a fraction equals
- * multiplication with the inverted fraction, and scale up both the
- * numerator and denominator with 8 to support at most 64 bits per IRQ:
+ * (addr & mask) gives us the _byte_ offset for the INT ID.
+ * We multiply this by 8 the get the _bit_ offset, then divide this by
+ * the number of bits to learn the actual INT ID.
+ * But instead of a division (which requires a "long long div" implementation),
+ * we shift by the binary logarithm of <bits>.
+ * This assumes that <bits> is a power of two.
*/
#define VGIC_ADDR_TO_INTID(addr, bits) (((addr) & VGIC_ADDR_IRQ_MASK(bits)) * \
- 64 / (bits) / 8)
+ 8 >> ilog2(bits))
/*
* Some VGIC registers store per-IRQ information, with a different number