Re: [RFC PATCH 00/13] x86 User Interrupts support

[Andy Lutomirski]

On Mon, Sep 13, 2021, at 1:01 PM, Sohil Mehta wrote:
> User Interrupts Introduction
> ============================
>
> User Interrupts (Uintr) is a hardware technology that enables delivering
> interrupts directly to user space.
>
> Today, virtually all communication across privilege boundaries happens by going
> through the kernel. These include signals, pipes, remote procedure calls and
> hardware interrupt based notifications. User interrupts provide the foundation
> for more efficient (low latency and low CPU utilization) versions of these
> common operations by avoiding transitions through the kernel.
>

...

I spent some time reviewing the docs (ISE) and contemplating how this all fits together, and I have a high level question:

Can someone give an example of a realistic workload that would benefit from SENDUIPI and precisely how it would use SENDUIPI?  Or an example of a realistic workload that would benefit from hypothetical device-initiated user interrupts and how it would use them?  I'm having trouble imagining something that wouldn't work as well or better by simply polling, at least on DMA-coherent architectures like x86.

(I can imagine some benefit to a hypothetical improved SENDUIPI with idential user semantics but that supported a proper interaction with the scheduler and blocking syscalls.  But that's not what's documented in the ISE...)

--Andy