Driver interface

EVL heavily relies on Dovetail for its seamless integration into the main kernel. Among other things, the latter extends the set of file operations which can be supported by file descriptions, so that they can handle I/O operations from the out-of-band stage upon request from an autonomous core such as EVL, while retaining the common structure of a character device driver. Applications running in user-space can start these I/O operations, by issuing specific system calls to EVL.

Excerpt from include/linux/fs.h, as modified by Dovetail

struct file_operations {
	...
	ssize_t (*oob_read) (struct file *, char __user *, size_t);
	ssize_t (*oob_write) (struct file *, const char __user *, size_t);
	long (*oob_ioctl) (struct file *, unsigned int, unsigned long);
	__poll_t (*oob_poll) (struct file *, struct oob_poll_wait *);
	...
} __randomize_layout;

Therefore, EVL drivers are common character device drivers, which only need to implement some out-of-band handlers for performing real-time I/O operations upon request from the EVL core. In other words, there is no such thing as an EVL driver model, because EVL fits into the regular Linux driver model.

Dovetail is in charge of routing the system calls received from applications to the proper recipient, either the EVL core or the host kernel. As mentioned earlier when describing the everything-is-a-file mantra, only I/O transfer and control requests have to run from the out-of-band context (i.e. EVL’s real-time mode), creating and dismantling the underlying file from the regular in-band context is just fine. Therefore, the execution flow upon I/O system calls looks like this:

Alt text

Which translates as follows:

  • When an application issues the open(2) or close(2) system call to an EVL driver, the request goes through the virtual filesystem switch (VFS) as usual, ending up into the .open() and .release() handlers (when the last reference is dropped) defined by the driver in its struct file_operations descriptor. The same goes for mmap(2), ioctl(2), read(2), write(2), and all other common file operations for character device drivers.

  • When an applications issues a oob_read(), oob_write() or oob_ioctl() system call via the EVL library, Dovetail routes the call to the EVL core (instead of the VFS), which in turn fires the corresponding handlers defined by the driver’s struct file_operations descriptor: i.e. .oob_read(), .oob_write(), and .oob_ioctl(). Those handlers should use the EVL kernel API, or the main kernel services which are known to be usable from the out-of-band-context, exclusively. Failing to abide by this golden rule may lead to funky outcomes ranging from plain kernel crashes (lucky) to rampant memory corruption (unlucky).

Now, you may wonder: “what if an out-of-band operation is ongoing in the driver on a particular file, while I’m closing the last VFS-maintained reference to that file?” Well, the close(2) call will block until the out-of-band operation finishes, at which point the .release() handler may proceed with dismantling the file. A simple rule for writing EVL drivers ensures this.