Re: [PATCH V3 10/10] x86/pks: Add PKS test code

[Dave Hansen]

On 11/6/20 3:29 PM, ira.weiny@xxxxxxxxx wrote:
> +		/* Arm for context switch test */
> +		write(fd, "1", 1);
> +
> +		/* Context switch out... */
> +		sleep(4);
> +
> +		/* Check msr restored */
> +		write(fd, "2", 1);

These are always tricky.  What you ideally want here is:

1. Switch away from this task to a non-PKS task, or
2. Switch from this task to a PKS-using task, but one which has a
   different PKS value

then, switch back to this task and make sure PKS maintained its value.

*But*, there's no absolute guarantee that another task will run.  It
would not be totally unreasonable to have the kernel just sit in a loop
without context switching here if no other tasks can run.

The only way you *know* there is a context switch is by having two tasks
bound to the same logical CPU and make sure they run one after another.
 This just gets itself into a state where it *CAN* context switch and
prays that one will happen.

You can also run a bunch of these in parallel bound to a single CPU.
That would also give you higher levels of assurance that *some* context
switch happens at sleep().

One critical thing with these tests is to sabotage the kernel and then
run them and make *sure* they fail.  Basically, if you screw up, do they
actually work to catch it?