linux-kernel - Re: [PATCH v2 6/7] x86/power: Sprinkle some noinstr

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Message-ID: <Y8aGpHgSOczqeEHf@hirez.programming.kicks-ass.net>
Date:   Tue, 17 Jan 2023 12:29:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     x86@...nel.org, Joan Bruguera <joanbrugueram@...il.com>,
        linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Jan Beulich <jbeulich@...e.com>,
        Roger Pau Monne <roger.pau@...rix.com>,
        Kees Cook <keescook@...omium.org>, mark.rutland@....com,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Jörg Rödel <joro@...tes.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 6/7] x86/power: Sprinkle some noinstr

On Tue, Jan 17, 2023 at 10:31:05AM +0100, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > +	/*
> > +	 * Definitely wrong, but at this point we should have at least enough
> > +	 * to do CALL/RET (consider SKL callthunks) and this avoids having
> > +	 * to deal with the noinstr explosion for now :/
> > +	 */
> > +	instrumentation_begin();
> 
> BTW., readability side note: instrumentation_begin()/end() are the 
> misnomers of the century - they don't signal the start/end of instrumented 
> code areas like the name falsely & naively suggests, but the exact 
> opposite: start/end of *non-*instrumented code areas.

Nope, they do as they say on the tin.

noinstr void foo(void)
{
}

declares the whole function as non-instrumented.

Within such functions, we demark regions where instrumentation is
allowed by:

noinstr void foo(void)
{
	instrumentation_begin();
	/* code that calls non-noinstr functions goes here */
	instrumentation_end();
}

(note the double negative in the comment)