Linux Interrupt Handling in User Space

问题:

In Linux, what are the options for handling device interrupts in user space code rather than in kernel space?

回答1:

Experience tells it is possible to write good and stable user-space drivers for almost any PCI adapter. It just requires some sophistication and a small proxying layer in the kernel. UIO is a step in that direction, but If you want to correctly handle interrupts in user-space then UIO might not be enough, for example if the device doesn't support the PCI-spec's interrupt disable bit which UIO relies on.

Notice that process wakeup latencies are a few microsecs so if your implementation requires very low latency then user-space might be a drag on it.

If I were to implement a user-space driver, I would reduce the kernel ISR to just a "disable & ack & wakeup-userpace" operation, handle the interrupt inside the waked-up process, and then re-enable the interrupt (of course, by writing to mapped PCI memory from the userspace process).

回答2:

There is UIO, but handling should still be done in kernelspace. OTOH, if you just need to notice the interrupt, you don't need the kernel part.

回答3:

You may like to take a look at CHAPTER 10: Interrupt Handling from Linux Device Drivers, Third Edition book.

回答4:

Have to trigger userland code indirectly.

Kernel ISR indicates interrupt by writing file / setting register / signalling. User space application polls this and goes on with the appropriate code. Edge cases: more or less interrupts than expected (time out / too many interrupts per time interval)

Linux file abstraction is used to connect kernel and user space. This is performed by character devices and ioctl() calls. Some may prefer sysfs entries for this purpose.

This can look odd because event triggered device notifications (interrupts) are hooked with 'time triggered' polling, but it is actually asyncronous blocking (read/select). Anyway some questions are arising according to performance.

So interrupts cannot be directly handled outside the kernel. E.g. shared memory can be in user space and with some I/O permission settings addresses can be mapped, so U-I/O works, but not for direct interrupt handling.

I have found only one 'minority report' in topic vfio (http://lxr.free-electrons.com/source/Documentation/vfio.txt): https://stackoverflow.com/a/21197797/5349798

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