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Date:   Tue, 19 Feb 2019 10:44:38 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "H. Peter Anvin" <hpa@zytor.com>
Cc:     Andy Lutomirski <luto@amacapital.net>,
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Subject: Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch
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On Tue, Feb 19, 2019 at 10:04:09AM +0100, Peter Zijlstra wrote:
> > Does that make more sense?
> 
> It appears to me you're going about it backwards.

So how about you do a GCC plugin that verifies limits on code-gen
between user_access_begin/user_access_end() ?

 - No CALL/RET
   - implies user_access_end() happens
   - implies no fentry hooks
 - No __preempt_count frobbing
 - No tracepoints
 - ...

That way you put the burden on the special code, not on the rest of the
kernel.