Simplifying a lot, both the CPU and the GPU have access to a few regions of shared memory, which both can read from and write to. Some of it is on the GPU (devi

Simplifying a lot, both the CPU and the GPU have access to a few regions of shared memory, which both can read from and write to. Some of it is on the GPU (device local memory) and some of it is on the CPU (host local memory). The CPU writes a list of commands to one of these shared memory regions, then writes to a special register on the GPU (these GPU registers are visible to the CPU as yet another region of shared memory) telling it where that list of commands is. That is, from a x86-64 assembly point of view, it's just writing to memory. Usually, only the final write to the special register is privileged, so that write will need an OS syscall into the graphics driver; the rest can be written directly by the userspace driver (which used beforehand another OS syscall to get access to some of that memory shared with the GPU). The userspace driver also compiles the SPIR-V programs into native GPU code, which can be loaded and executed as instructed by the list of commands.