IPC
Interprocess communication provides a way for thread of execution to cross Protection Domain (PD) boundaries.
The Vesper microkernel provides a message-passing IPC mechanism for communication between threads. The same mechanism is also used for communication with kernel-provided services. Messages are sent by invoking a capability to a kernel object. Messages sent to Endpoint are destined for other threads, while messages sent to other objects are processed by the kernel.
IPC mechanism allows for sharing some area of memory between kernel and one or more user applications, to avoid costly memory copy-in and copy-out.
Basic IPC is done via syscalls:
- Send
- send to a capability
- Non-blocking Send
- non-blocking send to a capability
- Call
- call a capability (send followed by recv?)
- Recv
- receive from an endpoint
- Non-blocking Recv
- receive but don't block if nothing to receive
- Reply
- send to a one-off reply capability
- Reply-then-Recv
- send reply atomically followed by a recv
These are performed on capabilities.
Additional kernel operations are implemented as calling certain exposed capabilities.
Logically, the kernel provides three system calls, Send, Receive and Yield. However, there are also combinations and variants of the basic Send and Receive calls, e.g. the Call operation, which consists of a send followed by a Receive from the same object. Methods on kernel objects other than endpoints and notifications are all mapped to Send or Call, depending on whether or not the method returns a result. The Yield system call is not associated with any kernel object and is the only operation that does not invoke a capability.
Endpoints allow a small amount of data and capabilities (namely the IPC buffer) to be transferred between two threads. Endpoint objects are invoked directly using the system calls described above.
IPC Endpoints use a rendezvous model and as such are synchronous and blocking. An Endpoint object may queue threads either to send or to receive. If no receiver is ready, threads performing the Send or Call system calls will wait in a queue for the first available receiver. Likewise, if no sender is ready, threads performing the Recv system call or the second half of ReplyRecv will wait for the first available sender.
Messages may contain capabilities, which will be transferred to the receiver, provided that the endpoint capability invoked by the sending thread has Grant rights. An attempt to send capabilities using an endpoint capability without the Grant right will result in transfer of the raw message, without any capability transfer.
