netdev - Re: [PATCH 3/3] powerpc: bpf: implement in-register swap for 64-bit endian operations

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Date:   Tue, 24 Jan 2017 21:55:11 +0530
From:   "'Naveen N. Rao'" <naveen.n.rao@...ux.vnet.ibm.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Benjamin Herrenschmidt <benh@....ibm.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "ast@...com" <ast@...com>,
        Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 3/3] powerpc: bpf: implement in-register swap for 64-bit
 endian operations

On 2017/01/24 04:13PM, David Laight wrote:
> From: 'Naveen N. Rao'
> > Sent: 23 January 2017 19:22
> > On 2017/01/15 09:00AM, Benjamin Herrenschmidt wrote:
> > > On Fri, 2017-01-13 at 23:22 +0530, 'Naveen N. Rao' wrote:
> > > > > That rather depends on whether the processor has a store to load forwarder
> > > > > that will satisfy the read from the store buffer.
> > > > > I don't know about ppc, but at least some x86 will do that.
> > > >
> > > > Interesting - good to know that.
> > > >
> > > > However, I don't think powerpc does that and in-register swap is likely
> > > > faster regardless. Note also that gcc prefers this form at higher
> > > > optimization levels.
> > >
> > > Of course powerpc has a load-store forwarder these days, however, I
> > > wouldn't be surprised if the in-register form was still faster on some
> > > implementations, but this needs to be tested.
> > 
> > Thanks for clarifying! To test this, I wrote a simple (perhaps naive)
> > test that just issues a whole lot of endian swaps and in _that_ test, it
> > does look like the load-store forwarder is doing pretty well.
> ...
> > This is all in a POWER8 vm. On POWER7, the in-register variant is around
> > 4 times faster than the ldbrx variant.
> ...
> 
> I wonder which is faster on the little 1GHz embedded ppc we use here.

Worth a test, for sure.
FWIW, this patch won't matter since eBPF JIT is for ppc64.

Thanks,
Naveen