netdev - Re: [PATCH] ppc: Fix BPF JIT for ABIv2

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Date:	Wed, 22 Jun 2016 20:27:25 +0530
From:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:	Thadeu Lima de Souza Cascardo <cascardo@...hat.com>
Cc:	Michael Ellerman <mpe@...erman.id.au>,
	Matt Evans <matt@...abs.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Alexei Starovoitov <ast@...com>, netdev@...r.kernel.org,
	Denis Kirjanov <kda@...ux-powerpc.org>,
	linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org,
	"David S. Miller" <davem@...emloft.net>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>
Subject: Re: [PATCH] ppc: Fix BPF JIT for ABIv2

On 2016/06/22 12:42PM, Naveen N Rao wrote:
> On 2016/06/21 11:47AM, Thadeu Lima de Souza Cascardo wrote:
> > On Tue, Jun 21, 2016 at 09:15:48PM +1000, Michael Ellerman wrote:
> > > On Tue, 2016-06-21 at 14:28 +0530, Naveen N. Rao wrote:
> > > > On 2016/06/20 03:56PM, Thadeu Lima de Souza Cascardo wrote:
> > > > > On Sun, Jun 19, 2016 at 11:19:14PM +0530, Naveen N. Rao wrote:
> > > > > > On 2016/06/17 10:00AM, Thadeu Lima de Souza Cascardo wrote:
> > > > > > > 
> > > > > > > Hi, Michael and Naveen.
> > > > > > > 
> > > > > > > I noticed independently that there is a problem with BPF JIT and ABIv2, and
> > > > > > > worked out the patch below before I noticed Naveen's patchset and the latest
> > > > > > > changes in ppc tree for a better way to check for ABI versions.
> > > > > > > 
> > > > > > > However, since the issue described below affect mainline and stable kernels,
> > > > > > > would you consider applying it before merging your two patchsets, so that we can
> > > > > > > more easily backport the fix?
> > > > > > 
> > > > > > Hi Cascardo,
> > > > > > Given that this has been broken on ABIv2 since forever, I didn't bother 
> > > > > > fixing it. But, I can see why this would be a good thing to have for 
> > > > > > -stable and existing distros. However, while your patch below may fix 
> > > > > > the crash you're seeing on ppc64le, it is not sufficient -- you'll need 
> > > > > > changes in bpf_jit_asm.S as well.
> > > > > 
> > > > > Hi, Naveen.
> > > > > 
> > > > > Any tips on how to exercise possible issues there? Or what changes you think
> > > > > would be sufficient?
> > > > 
> > > > The calling convention is different with ABIv2 and so we'll need changes 
> > > > in bpf_slow_path_common() and sk_negative_common().
> > > 
> > > How big would those changes be? Do we know?

So, this does need quite a few changes:
- the skb helpers need to emit code to setup TOC and the JIT code needs 
  to be updated to setup r12.
- the slow path code needs to be changed to store r3 elsewhere on ABIv2
- the above also means we need to change the stack macros with the 
  proper ABIv2 values
- the little endian support isn't complete as well -- some of the skb 
  helpers are not using byte swap instructions.

As such, I think we should just disable classic JIT on ppc64le.


- Naveen