linux-kernel - Re: [RFC][PATCH] tracing/x86: Save CR2 before tracing irqsoff on error

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Date:   Thu, 21 Mar 2019 10:02:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...capital.net>,
        Joel Fernandes <joel@...lfernandes.org>,
        He Zhe <zhe.he@...driver.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH] tracing/x86: Save CR2 before tracing irqsoff on
 error_entry

On Thu, Mar 21, 2019 at 09:33:17AM +0100, Peter Zijlstra wrote:

I'm thinking this problem wasn't new.

> idtentry page_fault             do_page_fault           has_error_code=1
>   call error_entry
>     TRACE_IRQS_OFF
>       call trace_hardirqs_off*
>         <tracer stuff>
> 	  <fault> # modifies CR2

      CALL_enter_from_user_mode
        __context_tracking_exit()
	  trace_user_exit(0)
	    #PF

>   call do_page_fault
>     address = read_cr2(); /* whoopsie */

And that also isn't fixed by your patch.

I'm trying to make idtentry put cr2 in rdx, such that do_page_fault()
takes address as a 3rd argument, but I'm still fighting that context
tracking nonsense.