vhost

overlay/virtqueue/size.go

@@ -0,0 +1,33 @@
+package virtqueue
+
+import (
+	"errors"
+	"fmt"
+)
+
+// ErrQueueSizeInvalid is returned when a queue size is invalid.
+var ErrQueueSizeInvalid = errors.New("queue size is invalid")
+
+// CheckQueueSize checks if the given value would be a valid size for a
+// virtqueue and returns an [ErrQueueSizeInvalid], if not.
+func CheckQueueSize(queueSize int) error {
+	if queueSize <= 0 {
+		return fmt.Errorf("%w: %d is too small", ErrQueueSizeInvalid, queueSize)
+	}
+
+	// The queue size must always be a power of 2.
+	// This ensures that ring indexes wrap correctly when the 16-bit integers
+	// overflow.
+	if queueSize&(queueSize-1) != 0 {
+		return fmt.Errorf("%w: %d is not a power of 2", ErrQueueSizeInvalid, queueSize)
+	}
+
+	// The largest power of 2 that fits into a 16-bit integer is 32768.
+	// 2 * 32768 would be 65536 which no longer fits.
+	if queueSize > 32768 {
+		return fmt.Errorf("%w: %d is larger than the maximum possible queue size 32768",
+			ErrQueueSizeInvalid, queueSize)
+	}
+
+	return nil
+}