linux-kernel - Re: [PATCH 0/3] System call table generation support

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Message-ID: <20180921193216.wta4bvmmbp2jyasl@pburton-laptop>
Date:   Fri, 21 Sep 2018 19:32:18 +0000
From:   Paul Burton <paul.burton@...s.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     Arnd Bergmann <arnd@...db.de>, Firoz Khan <firoz.khan@...aro.org>,
        Hauke Mehrtens <hauke@...ke-m.de>,
        Rafał Miłecki <zajec5@...il.com>,
        "open list:RALINK MIPS ARCHITECTURE" <linux-mips@...ux-mips.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        gregkh <gregkh@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Marcin Juszkiewicz <marcin.juszkiewicz@...aro.org>
Subject: Re: [PATCH 0/3] System call table generation support

Hi Christoph,

On Thu, Sep 20, 2018 at 11:09:41PM -0700, Christoph Hellwig wrote:
> On Thu, Sep 20, 2018 at 08:48:37PM +0000, Paul Burton wrote:
> > > Speaking of nanoMIPS, what is your plan for the syscall ABI there?
> > > I can see two ways of approaching it:
> > > 
> > > a) keep all the MIPSisms in the data structures, and just use a subset of
> > >     o32 that drops all the obsolete entry points
> > > b) start over and stay as close as possible to the generic ABI, using the
> > >     asm-generic versions of both the syscall table and the uapi header
> > >     files instead of the traditional version.
> > 
> > We've taken option b in our current downstream kernel & that's what I
> > hope we'll get upstream too. There's no expectation that we'll ever need
> > to mix pre-nanoMIPS & nanoMIPS ISAs or their associated ABIs across the
> > kernel/user boundary so it's felt like a great opportunity to clean up &
> > standardise.
> > 
> > Getting nanoMIPS/p32 support submitted upstream is on my to-do list, but
> > there's a bunch of prep work to get in first & of course that to-do list
> > is forever growing. Hopefully in the next couple of cycles.
> 
> p32 is just the ABI name for nanoMIPS or yet another MIPS ABI?

p32 is the ABI for nanoMIPS - ie. it is a new ABI, but it's not for use
with pre-nanoMIPS ISAs & nanoMIPS isn't used with o32/n32/n64.

Some of the code density improvements nanoMIPS brings are due to the ISA
& p32 ABI being developed together - eg. the load/store multiple &
save/restore instructions make it easy to save sequences of $sp, $fp,
$ra & some number of the $sN callee-saved registers. Compressed register
number encodings generally include registers that make sense for the p32
ABI, and I'm sure there were other things I've forgotten.

> Either way, І think if there is yet another ABI even on an existing port
> we should always aim for the asm-generic syscall table indeed.
> 
> Especially for mips where o32 has a rather awkward ABI only explained by
> odd decisions more than 20 years ago.

Glad to hear we're on the same page :)

I'm all for being less "special" & couldn't care less if our nanoMIPS
support isn't compatible with IRIX.

Thanks,
    Paul