linux-kernel - Re: [PATCH 3/5] MIPS: Only change $28 to thread

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Date:   Mon, 5 Dec 2016 17:27:11 +0000
From:   Paul Burton <paul.burton@...tec.com>
To:     "Maciej W. Rozycki" <macro@...tec.com>
CC:     Matt Redfearn <matt.redfearn@...tec.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        <linux-mips@...ux-mips.org>,
        "Jason A . Donenfeld" <Jason@...c4.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        <linux-kernel@...r.kernel.org>,
        James Hogan <james.hogan@...tec.com>
Subject: Re: [PATCH 3/5] MIPS: Only change $28 to thread_info if coming from user mode

Hi Maciej,

>  Overall I think all <asm/stackframe.h> code should be using the (default)
> `.set reorder' mode, perhaps forced explicitly in case these macros are
> pasted into `.set noreorder' code, to make it easier to avoid subtle data
> dependency bugs, and also to make R6 porting easier.  Except maybe for the
> RFE sequence, for readability's sake, although even there GAS will do the
> right thing.  Surely the BLTZ/MOVE piece does not have to be `.set
> noreorder' as GAS will schedule that delay slot automatically if allowed
> to.

Agreed we ought to use .set reorder (or rather, not use .set noreorder) 
wherever possible but FYI one thing I've only noticed recently is that we 
don't actually get any reordering anyway, presumably because we don't provide 
any -O flags when building asm source. I haven't yet done the legwork or 
figuring out whether we've ever had optimisation & if so what changed...

Thanks,
    Paul
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