linux-kernel - Re: [patch V4 part 4 15/24] x86/db: Split out dr6/7 handling

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Date:   Thu, 14 May 2020 13:46:10 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
        paulmck <paulmck@...nel.org>, Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>, rostedt <rostedt@...dmis.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch V4 part 4 15/24] x86/db: Split out dr6/7 handling

----- On May 14, 2020, at 1:28 PM, Thomas Gleixner tglx@...utronix.de wrote:

> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> writes:
>> ----- On May 5, 2020, at 9:49 AM, Thomas Gleixner tglx@...utronix.de wrote:
>>> +
>>> +static __always_inline void debug_exit(unsigned long dr7)
>>> +{
>>> +	set_debugreg(dr7, 7);
>>> +}
>>

* Question 1

>> Out of curiosity, what prevents the compiler from moving instructions
>> outside of the code regions surrounded by entry/exit ? This is an always
>> inline, which invokes set_debugreg which is inline for CONFIG_PARAVIRT_XXL=n,
>> which in turn uses an asm() (not volatile), without any memory
>> clobber.
>>

?

* Question 2

>> Also, considering that "inline" is not sufficient to ensure the compiler
>> does not emit a traceable function, I suspect you'll also want to mark
>> "native_get_debugreg" and "native_set_debugreg" always inline as well.
> 
> It can move it into a function and call that. Fine. If that function
> stays in the noinstr section then it should not emit a trace stub and if
> it moves it out of the section or reuses another instance in text then
> objtool will complain.
> 
> Checking for trace stubs and other instrumentation nonsense is on the
> objtool wishlist anyway.
> 
> But yes, marking these __always_inline prevents that. Those escaped my
> chase. But I would have found them once I go and fix that paravirt muck.

This answers only my second question (see "Question 1" above).

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com