linux-kernel - Re: [PATCH] powerpc/32: Clear volatile regs on syscall exit

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Date:   Wed, 23 Feb 2022 17:27:39 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Gabriel Paubert <paubert@...m.es>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/32: Clear volatile regs on syscall exit

On Wed, Feb 23, 2022 at 09:48:09PM +0100, Gabriel Paubert wrote:
> On Wed, Feb 23, 2022 at 06:11:36PM +0100, Christophe Leroy wrote:
> > +	/* Zero volatile regs that may contain sensitive kernel data */
> > +	li	r0,0
> > +	li	r4,0
> > +	li	r5,0
> > +	li	r6,0
> > +	li	r7,0
> > +	li	r8,0
> > +	li	r9,0
> > +	li	r10,0
> > +	li	r11,0
> > +	li	r12,0
> > +	mtctr	r0
> > +	mtxer	r0
> 
> Here, I'm almost sure that on some processors, it would be better to
> separate mtctr form mtxer. mtxer is typically very expensive (pipeline
> flush) but I don't know what's the best ordering for the average core.

mtxer is cheaper than mtctr on many cores :-)

On p9 mtxer is cracked into two latency 3 ops (which run in parallel).
While mtctr has latency 5.

On p8 mtxer was horrible indeed (but nothing near as bad as a pipeline
flush).


Segher