kvm tools: Add helper to retrieve the field used in virtio config space

This patch adds a helper used to retrieve the type of field used when guest
is writing or reading from virtio config space.

Since the config space is dynamic, it may change during runtime - so we must
calculate it before every read/write.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

include/kvm/virtio.h

@@ -12,6 +12,10 @@
 #define VIRTIO_IRQ_LOW		0
 #define VIRTIO_IRQ_HIGH		1
 
+#define VIRTIO_PCI_O_CONFIG	0
+#define VIRTIO_PCI_O_MSIX	1
+#define VIRTIO_PCI_O_FEATURES	2
+
 struct virt_queue {
 	struct vring	vring;
 	u32		pfn;
@@ -56,4 +60,6 @@ u16 virt_queue__get_inout_iov(struct kvm *kvm, struct virt_queue *queue,
 
 void virt_queue__trigger_irq(struct virt_queue *vq, int irq, u8 *isr, struct kvm *kvm);
 
+int virtio__get_dev_specific_field(int offset, bool msix, bool features_hi, u32 *config_off);
+
 #endif /* KVM__VIRTIO_H */

virtio/core.c

@@ -100,3 +100,24 @@ void virt_queue__trigger_irq(struct virt_queue *vq, int irq, u8 *isr, struct kvm
 		kvm__irq_line(kvm, irq, VIRTIO_IRQ_HIGH);
 	}
 }
+
+int virtio__get_dev_specific_field(int offset, bool msix, bool features_hi, u32 *config_off)
+{
+	if (msix) {
+		if (offset < 4)
+			return VIRTIO_PCI_O_MSIX;
+		else
+			offset -= 4;
+	}
+
+	if (features_hi) {
+		if (offset < 4)
+			return VIRTIO_PCI_O_FEATURES;
+		else
+			offset -= 4;
+	}
+
+	*config_off = offset;
+
+	return VIRTIO_PCI_O_CONFIG;
+}