[PATCH kvmtool 2/4] virtio/mmio: access header members normally

[Andre Przywara <andre.przywara@xxxxxxx>]

The handlers for accessing the virtio-mmio header tried to be very
clever, by modelling the internal data structure to look exactly like
the protocol header, so that address offsets can "reused".

This requires using a packed structure, which creates other problems,
and seems to be totally unnecessary in this case.

Replace the offset-based access hacks to the structure with proper
compiler visible accesses, to avoid unaligned accesses and make the code
more robust.

This fixes UBSAN complaints about unaligned accesses.

Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
---
 include/kvm/virtio-mmio.h |  2 +-
 virtio/mmio.c             | 19 +++++++++++++++----
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/include/kvm/virtio-mmio.h b/include/kvm/virtio-mmio.h
index 13dcccb6..aa4cab3c 100644
--- a/include/kvm/virtio-mmio.h
+++ b/include/kvm/virtio-mmio.h
@@ -39,7 +39,7 @@ struct virtio_mmio_hdr {
 	u32	interrupt_ack;
 	u32	reserved_5[2];
 	u32	status;
-} __attribute__((packed));
+};
 
 struct virtio_mmio {
 	u32			addr;
diff --git a/virtio/mmio.c b/virtio/mmio.c
index 3782d55a..c9ad8ee7 100644
--- a/virtio/mmio.c
+++ b/virtio/mmio.c
@@ -135,12 +135,22 @@ static void virtio_mmio_config_in(struct kvm_cpu *vcpu,
 
 	switch (addr) {
 	case VIRTIO_MMIO_MAGIC_VALUE:
+		memcpy(data, &vmmio->hdr.magic, sizeof(vmmio->hdr.magic));
+		break;
 	case VIRTIO_MMIO_VERSION:
+		ioport__write32(data, vmmio->hdr.version);
+		break;
 	case VIRTIO_MMIO_DEVICE_ID:
+		ioport__write32(data, vmmio->hdr.device_id);
+		break;
 	case VIRTIO_MMIO_VENDOR_ID:
+		ioport__write32(data, vmmio->hdr.vendor_id);
+		break;
 	case VIRTIO_MMIO_STATUS:
+		ioport__write32(data, vmmio->hdr.status);
+		break;
 	case VIRTIO_MMIO_INTERRUPT_STATUS:
-		ioport__write32(data, *(u32 *)(((void *)&vmmio->hdr) + addr));
+		ioport__write32(data, vmmio->hdr.interrupt_state);
 		break;
 	case VIRTIO_MMIO_DEVICE_FEATURES:
 		if (vmmio->hdr.host_features_sel == 0)
@@ -174,9 +184,10 @@ static void virtio_mmio_config_out(struct kvm_cpu *vcpu,
 
 	switch (addr) {
 	case VIRTIO_MMIO_DEVICE_FEATURES_SEL:
+		vmmio->hdr.host_features_sel = ioport__read32(data);
+		break;
 	case VIRTIO_MMIO_DRIVER_FEATURES_SEL:
-		val = ioport__read32(data);
-		*(u32 *)(((void *)&vmmio->hdr) + addr) = val;
+		vmmio->hdr.guest_features_sel = ioport__read32(data);
 		break;
 	case VIRTIO_MMIO_QUEUE_SEL:
 		val = ioport__read32(data);
@@ -185,7 +196,7 @@ static void virtio_mmio_config_out(struct kvm_cpu *vcpu,
 				val, vq_count);
 			break;
 		}
-		*(u32 *)(((void *)&vmmio->hdr) + addr) = val;
+		vmmio->hdr.queue_sel = val;
 		break;
 	case VIRTIO_MMIO_STATUS:
 		vmmio->hdr.status = ioport__read32(data);
-- 
2.25.1