linux-kernel - Re: [PATCH v3 04/22] x86/doublefault: Make memmove() notrace/NOKPROBE

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Date:   Wed, 19 Feb 2020 16:40:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org, joel@...lfernandes.org,
        gregkh@...uxfoundation.org, gustavo@...eddedor.com,
        tglx@...utronix.de, paulmck@...nel.org, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        luto@...nel.org, tony.luck@...el.com, frederic@...nel.org,
        dan.carpenter@...cle.com, mhiramat@...nel.org
Subject: Re: [PATCH v3 04/22] x86/doublefault: Make memmove() notrace/NOKPROBE

On Wed, Feb 19, 2020 at 10:36:14AM -0500, Steven Rostedt wrote:
> On Wed, 19 Feb 2020 15:47:28 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > --- a/arch/x86/lib/memcpy_32.c
> > +++ b/arch/x86/lib/memcpy_32.c
> > @@ -21,7 +21,7 @@ __visible void *memset(void *s, int c, s
> >  }
> >  EXPORT_SYMBOL(memset);
> >  
> > -__visible void *memmove(void *dest, const void *src, size_t n)
> > +__visible notrace void *memmove(void *dest, const void *src, size_t n)
> >  {
> >  	int d0,d1,d2,d3,d4,d5;
> >  	char *ret = dest;
> > @@ -207,3 +207,8 @@ __visible void *memmove(void *dest, cons
> >  
> >  }
> >  EXPORT_SYMBOL(memmove);
> 
> Hmm, for things like this, which is adding notrace because of a single
> instance of it (although it is fine to trace in any other instance), it
> would be nice to have a gcc helper that could call "memmove+5" which
> would skip the tracing portion.

Or just open-code the memmove() in do_double_fault() I suppose. I don't
think we care about super optimized code there. It's the bloody ESPFIX
trainwreck.