refactoring a bit

overlay/vhostnet/device.go

@@ -223,10 +223,7 @@ func (dev *Device) GetPacketForTx() (uint16, []byte, error) {
 	if err != nil {
 		return 0, nil, fmt.Errorf("transmit queue: %w", err)
 	}
-	buf, err := dev.TransmitQueue.GetDescriptorItem(idx)
-	if err != nil {
-		return 0, nil, fmt.Errorf("get descriptor chain: %w", err)
-	}
+	buf := dev.TransmitQueue.GetDescriptorItem(idx)
 	return idx, buf, nil
 }
 
@@ -273,10 +270,7 @@ func (dev *Device) ProcessRxChain(pkt *VirtIOPacket, chain virtqueue.UsedElement
 	//read first element to see how many descriptors we need:
 	pkt.Reset()
 	idx := uint16(chain.DescriptorIndex)
-	buf, err := dev.ReceiveQueue.GetDescriptorItem(idx)
-	if err != nil {
-		return 0, fmt.Errorf("get descriptor chain: %w", err)
-	}
+	buf := dev.ReceiveQueue.GetDescriptorItem(idx)
 
 	// The specification requires that the first descriptor chain starts
 	// with a virtio-net header. It is not clear, whether it is also
@@ -284,20 +278,25 @@ func (dev *Device) ProcessRxChain(pkt *VirtIOPacket, chain virtqueue.UsedElement
 	// descriptor chain, but it is reasonable to assume that this is
 	// always the case.
 	// The decode method already does the buffer length check.
-	if err = pkt.header.Decode(buf); err != nil {
-		// The device misbehaved. There is no way we can gracefully
-		// recover from this, because we don't know how many of the
-		// following descriptor chains belong to this packet.
-		return 0, fmt.Errorf("decode vnethdr: %w", err)
-	}
+
+	//HACK: we only want the last bit of the header, the NumBuffers field. So, let's grab just that:
+	//numBuffers := binary.BigEndian.Uint16(buf[virtio.NetHdrSize-3:])
+	//even bigger hack: apparently this is hitting some kind of memory access pitfall? Let's only grab the last byte:
+	//numBuffers := buf[virtio.NetHdrSize-2]
+
+	//if err = pkt.header.Decode(buf); err != nil {
+	//	// The device misbehaved. There is no way we can gracefully
+	//	// recover from this, because we don't know how many of the
+	//	// following descriptor chains belong to this packet.
+	//	return 0, fmt.Errorf("decode vnethdr: %w", err)
+	//}
 
 	//we have the header now: what do we need to do?
-	if int(pkt.header.NumBuffers) > 1 {
-		return 0, fmt.Errorf("number of buffers is greater than number of chains %d", 1)
-	}
-	if int(pkt.header.NumBuffers) != 1 {
-		return 0, fmt.Errorf("too smol-brain to handle more than one buffer per chain item right now: %d chains, %d bufs", 1, int(pkt.header.NumBuffers))
-	}
+	//todo we're ignoring the header lol
+	//if int(numBuffers) != 1 {
+	//	return 0, fmt.Errorf("too smol-brain to handle more than one buffer per Chain item right now: %d chains, %d bufs", 1, int(numBuffers))
+	//}
+
 	if chain.Length > 16000 {
 		//todo!
 		return 1, fmt.Errorf("too big packet length: %d", chain.Length)
@@ -311,7 +310,7 @@ func (dev *Device) ProcessRxChain(pkt *VirtIOPacket, chain virtqueue.UsedElement
 
 type VirtIOPacket struct {
 	payload []byte
-	header  virtio.NetHdr
+	//header  virtio.NetHdr
 	Chains []uint16
 }
 
@@ -328,8 +327,8 @@ func (v *VirtIOPacket) Reset() {
 }
 
 func (v *VirtIOPacket) GetPayload() []byte {
-	return v.payload
+	return v.payload //todo this could be dev.ReceiveQueue.GetDescriptorItem(idx)
 }
 func (v *VirtIOPacket) SetPayload(x []byte) {
-	v.payload = x //todo?
+	v.payload = x
 }

overlay/virtqueue/descriptor_table.go

@@ -268,18 +268,13 @@ func (dt *DescriptorTable) createDescriptorForInputs() (uint16, error) {
 	return head, nil
 }
 
-func (dt *DescriptorTable) getDescriptorItem(head uint16) ([]byte, error) {
-	if int(head) > len(dt.descriptors) {
-		return nil, fmt.Errorf("%w: index out of range", ErrInvalidDescriptorChain)
-	}
-
+func (dt *DescriptorTable) getDescriptorItem(head uint16) []byte {
 	desc := &dt.descriptors[head] //todo this is a pretty nasty hack with no checks
 
 	// The descriptor address points to memory not managed by Go, so this
 	// conversion is safe. See https://github.com/golang/go/issues/58625
 	//goland:noinspection GoVetUnsafePointer
-	bs := unsafe.Slice((*byte)(unsafe.Pointer(desc.address)), desc.length)
-	return bs, nil
+	return unsafe.Slice((*byte)(unsafe.Pointer(desc.address)), desc.length)
 }
 
 // checkUnusedDescriptorLength asserts that the length of an unused descriptor

overlay/virtqueue/split_virtqueue.go

@@ -340,7 +340,7 @@ func (sq *SplitQueue) OfferInDescriptorChains() (uint16, error) {
 // Be careful to only access the returned buffer slices when the device is no
 // longer using them. They must not be accessed after
 // [SplitQueue.FreeDescriptorChain] has been called.
-func (sq *SplitQueue) GetDescriptorItem(head uint16) ([]byte, error) {
+func (sq *SplitQueue) GetDescriptorItem(head uint16) []byte {
 	sq.descriptorTable.descriptors[head].length = uint32(sq.descriptorTable.itemSize)
 	return sq.descriptorTable.getDescriptorItem(head)
 }
@@ -350,13 +350,19 @@ func (sq *SplitQueue) SetDescSize(head uint16, sz int) {
 	sq.descriptorTable.descriptors[int(head)].length = uint32(sz)
 }
 
-func (sq *SplitQueue) OfferDescriptorChains(chains []uint16, kick bool) error {
-	//todo not doing this may break eventually?
-	//not called under lock
-	//if err := sq.descriptorTable.freeDescriptorChain(head); err != nil {
-	//	return fmt.Errorf("free: %w", err)
-	//}
+func (sq *SplitQueue) OfferDescriptor(chain uint16, kick bool) error {
+	// Make the descriptor chain available to the device.
+	sq.availableRing.offerSingle(chain)
 
+	// Notify the device to make it process the updated available ring.
+	if kick {
+		return sq.Kick()
+	}
+
+	return nil
+}
+
+func (sq *SplitQueue) OfferDescriptorChains(chains []uint16, kick bool) error {
 	// Make the descriptor chain available to the device.
 	sq.availableRing.offer(chains)