linux-kernel - Re: [PATCH] jffs2: implement mount option to configure endianness

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Date:   Wed, 7 Nov 2018 18:41:47 +0000
From:   Joakim Tjernlund <Joakim.Tjernlund@...inera.com>
To:     "danielwa@...co.com" <danielwa@...co.com>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "nkela@...co.com" <nkela@...co.com>,
        "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        "xe-linux-external@...co.com" <xe-linux-external@...co.com>
Subject: Re: [PATCH] jffs2: implement mount option to configure endianness

On Wed, 2018-11-07 at 17:58 +0000, David Woodhouse wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe.
> 
> 
> > On Wed, Nov 07, 2018 at 04:12:14PM -0000, David Woodhouse wrote:
> > > > Yes, this may slow things down. I am not sure I agree with the impl.
> > > > either.
> > > > Could one not make cpu_to_je_X/jeX_to_cpu a function ptr which is set
> > > to
> > > > a func. with the correct endian?
> > > 
> > > On x86 retpoline would make that quite slow.

retpoline aside, would it make a difference ?

> > 
> > Is x86 the largest consumer of jffs2 ?
> 
> Certainly not. I'm not sure which architectures do have Spectre V2
> mitigations which make indirect branches expensive now... perhaps there is
> no intersection with the cases where we really care about JFFS2 being
> CPU-bound?

All that passing of a new extra arg and extra if .. elif .. does cost too.