linux-kernel - Re: [NEEDS-REVIEW] [PATCH V3 04/10] x86/pks: Preserve the PKRS MSR on context switch

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Date:   Fri, 18 Dec 2020 07:33:03 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     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>,
        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: [NEEDS-REVIEW] [PATCH V3 04/10] x86/pks: Preserve the PKRS MSR on
 context switch

On 12/17/20 8:10 PM, Ira Weiny wrote:
> On Thu, Dec 17, 2020 at 12:41:50PM -0800, Dave Hansen wrote:
>> On 11/6/20 3:29 PM, ira.weiny@...el.com wrote:
>>>  void disable_TSC(void)
>>> @@ -644,6 +668,8 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p)
>>>  
>>>  	if ((tifp ^ tifn) & _TIF_SLD)
>>>  		switch_to_sld(tifn);
>>> +
>>> +	pks_sched_in();
>>>  }
>>
>> Does the selftest for this ever actually schedule()?
> 
> At this point I'm not sure.  This code has been in since the beginning.  So its
> seen a lot of soak time.

Think about it another way.  Let's say this didn't get called on the
first context switch away from the PKS-using task.  Would anyone notice?
 How likely is this to happen?

The function tracers or kprobes tend to be a great tool for this, at
least for testing whether the code path you expect to hit is getting hit.