Subject: Re: [PATCH] powerpc/32: Clear volatile regs on syscall exit

From: Segher Boessenkool <[email-masked]>
Date: Thu, 24 Feb 2022 06:49:11 -0600

Hi!

On Thu, Feb 24, 2022 at 09:29:55AM +0100, Gabriel Paubert wrote:
> On Wed, Feb 23, 2022 at 05:27:39PM -0600, Segher Boessenkool wrote:
> > 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 :-)
> 
> We're speaking of 32 bit here I believe;

32-bit userland, yes.  Which runs fine on non-ancient cores, too.

> on my (admittedly old) paper
> copy of PowerPC 604 user's manual, I read in a footnote:
> 
> "The mtspr (XER) instruction causes instructions to be flushed when it
> executes." 

And the 604 has a trivial depth pipeline anyway.

> I know there are probably very few 604 left in the field, but in this
> case mtspr(xer) looks very much like a superset of isync.

It hasn't been like that for decades.  On the 750 mtxer was execution
synchronised only already, for example.

> I also just had a look at the documentation of a more widespread core:
> 
> https://www.nxp.com/docs/en/reference-manual/MPC7450UM.pdf
> 
> and mtspr(xer) is marked as execution and refetch serialized, actually
> it is the only instruction to have both.

This looks like a late addition (it messes up the table, for example,
being put after "mtspr (other)").  It also is different from 7400 and
750 and everything else.  A late bugfix?  Curious :-)

> Maybe there is a subtle difference between "refetch serialization" and
> "pipeline flush", but in this case please educate me.

There is a subtle difference, but it goes the other way: refetch
serialisation doesn't stop fetch / flush everything after it, only when
the instruction completes it rejects everything after it.  So it can
waste a bit more :-)

> Besides that the back to back mtctr/mtspr(xer) may limit instruction
> decoding and issuing bandwidth.

It doesn't limit decode or dispatch (not issue fwiw) bandwidth on any
core I have ever heard of.

> I'd rather move one of them up by a few
> lines since they can only go to one of the execution units on some
> (or even most?) cores. This was my main point initially.

I think it is much more beneficial to *not* do these insns than to
shift them back and forth a cycle.

Segher