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

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Date:   Thu, 17 Dec 2020 12:55:39 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     ira.weiny@...el.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-doc@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH V3 10/10] x86/pks: Add PKS test code

On 11/6/20 3:29 PM, ira.weiny@...el.com 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?