The lavapipe layer is a gallium frontend. It takes the Vulkan API and roughly translates it into the gallium API.
Vulkan is a lowlevel API, it allows the user to allocate memory, create resources, record command buffers amongst other things. When a hw vulkan driver is recording a command buffer, it is putting hw specific commands into it that will be run directly on the GPU. These command buffers are submitted to queues when the app wants to execute them.
Gallium is a context level API, i.e. like OpenGL/D3D10. The user has to create resources and contexts and the driver internally manages command buffers etc. The driver controls internal flushing and queuing of command buffers.
In order to bridge the gap, the lavapipe layer abstracts the gallium context into a separate thread of execution. When recording a vulkan command buffer it creates a CPU side command buffer containing an encoding of the Vulkan API. It passes that recorded CPU command buffer to the thread on queue submission. The thread then creates a gallium context, and replays the whole CPU recorded command buffer into the context, one command at a time.
Software rasterizers are a very different proposition from an overhead point of view than real hardware. CPU rasterization is pretty heavy on the CPU load, so nearly always 90% of your CPU time will be in the rasterizer and fragment shader. Having some minor CPU overheads around command submission and queuing isn't going to matter in the overall profile of the user application. CPU rasterization is already slow, the Vulkan->gallium translation overhead isn't going to be the reason for making it much slower.
For real HW drivers which are meant to record their own command buffers in the GPU domain and submit them direct to the hw, adding in a CPU layer that just copies the command buffer data is a massive overhead and one that can't easily be removed from the lavapipe layer.
The lavapipe execution context is also pretty horrible, it has to connect all the state pieces like shaders etc to the gallium context, and disconnect them all at the end of each command buffer. There is only one command submission queue, one context to be used. A lot of hardware exposes more queues etc that this will never model.
Pipeline barriers in Vulkan are essential to efficient driver hw usage. They are one of the most difficult to understand and hard to get right pieces of writing a vulkan driver. For a software rasterizer they are also mostly unneeded. When I get a barrier I just completely hardflush the gallium context because I know the sw driver behind it. For a real hardware driver this would be a horrible solution. You spend a lot of time trying to make anything optimal here.
