RE: [PATCH v8 06/14] task_isolation: provide strict mode configurable signal

[Gilad Ben Yossef]

> From: Andy Lutomirski [mailto:luto@xxxxxxxxxxxxxx]
> Sent: Wednesday, October 21, 2015 4:43 AM
> To: Chris Metcalf
> Subject: Re: [PATCH v8 06/14] task_isolation: provide strict mode
> configurable signal
> 
> On Tue, Oct 20, 2015 at 6:30 PM, Chris Metcalf <cmetcalf@xxxxxxxxxx>
> wrote:
> > On 10/20/2015 8:56 PM, Steven Rostedt wrote:
> >>
> >> On Tue, 20 Oct 2015 16:36:04 -0400
> >> Chris Metcalf <cmetcalf@xxxxxxxxxx> wrote:
> >>
> >>> Allow userspace to override the default SIGKILL delivered
> >>> when a task_isolation process in STRICT mode does a syscall
> >>> or otherwise synchronously enters the kernel.
> >>>
<snip>
> >
> > It doesn't map SIGKILL to some other signal unconditionally.  It just allows
> > the "hey, you broke the STRICT contract and entered the kernel" signal
> > to be something besides the default SIGKILL.
> >
> 

<snip>
> 
> I still dislike this thing.  It seems like a debugging feature being
> implemented using signals instead of existing APIs.  I *still* don't
> see why perf can't be used to accomplish your goal.
> 

It is not (just) a debugging feature. There are workloads were not performing an action is much preferred to being late.

Consider the following artificial but representative scenario: a task running in strict isolation is controlling a radiotherapy alpha emitter.
The code runs in a tight event loop, reading an MMIO register with location data, making some calculation and in response writing an 
MMIO register that triggers the alpha emitter. As a safety measure, each trigger is for a specific very short time frame - the alpha emitter 
auto stops.

The code has a strict assumption that no more than X cycles pass between reading the value and the response and the system is built in 
such a way that as long as the code has mastery of the CPU the assumption holds true. If something breaks this assumption (unplanned
context switch to kernel), what you want to do is just stop place
rather than fire the alpha emitter X nanoseconds too late.

This feature lets you say: if the "contract" of isolation is broken, notify/kill me at once.

For code where isolation is important, the correctness of a calculation is dependent on timing. It's like you would accept the kernel to
kill a task if it read from an unmapped virtual address rather than returning garbage data. With an isolated task, the right data acted on 
later than you think is garbage just the same.

I hope this sheds some light on the issue.

Thanks,
Gilad