Yeah, sure it is hard or even harder. But think about browsers. We run applications and we want to make sure they look and behave the same, but there are so many OS, browsers, versions, devices, ...
So do we have to buy a laptop for each? Maybe in the past, but today we "just" emulate it. The same can be done with kernel testing. Assume that the whole kernel runs emulated (which is the hard part) and then test the outout of that (in the emulator).
Browser and kernel development couldn't be more different.
> Maybe in the past, but today we "just" emulate it.
"Just emulating" is almost impossible. For testing to be actually reliable, you'd need to recreate all the hardware behavior in code. OS virtualization relies on re-exposing the hardware and kernel primitives to VMs, no VM is reimplementing the Intel architecture for memory management, for example.
> then test the outout of that (in the emulator).
Another hard topic. What's the output of a kernel? In an emulator the only thing you have is access to the memory areas (RAM, screen, devices). You can't really rebuild the state of the kernel, or access internal structures that tell you information about how it is. If you're lucky and advanced enough in the boot sequence that the kernel has configured the serial port for comms and debugging, you might be able to get some information, but not that much, and it'd still be impossible to test certain parts of the boot sequence.
> OS virtualization relies on re-exposing the hardware and kernel primitives to VMs, no VM is reimplementing the Intel architecture for memory management, for example.
That's fair. But in the same way that the VM relies on that the host OS handles operations correctly, the kernel can assume the same, as long as there is a clean separation (so that one kernel doesn't influence the other).
Of course, if you have a bug in the (old/other) kernel that influences the tests themselves then that wouldn't work, but I would say that this probably makes up a very small minority of cases.
Just like if Intel runs tests for their CPUs on other CPUs and those other CPUs are buggy...
> Another hard topic. What's the output of a kernel
The kernel has an interface and if I ask it to write something and then read it back, I have an expectation about what happens. I don't need to care about the state of the kernel (neither should I) as long as the result is good. Only for certain performance-relevant things I can see that this might be a problem.
> and it'd still be impossible to test certain parts of the boot sequence
I'm neither a kernel nor a browser developer ;) but I would be interested in an example. You are probably right, but I fail to see what that would be.
Yeah I think that's true. From my experience it's the same for testing software in general - you can mock things but having actual tests that test the behaviour of dependencies is tricky since you have to emulate those for your tests and that can be a lot of work.
But since it covers your for all future changes, it might still be worth it, because otherwise you either have to manually retest with actual hardware or accept a higher risk of bugs.
Yeah, sure it is hard or even harder. But think about browsers. We run applications and we want to make sure they look and behave the same, but there are so many OS, browsers, versions, devices, ...
So do we have to buy a laptop for each? Maybe in the past, but today we "just" emulate it. The same can be done with kernel testing. Assume that the whole kernel runs emulated (which is the hard part) and then test the outout of that (in the emulator).