linux-kernel - Re: [PATCH 3/8] asm-generic: fix unaligned access hamdling in raw_copy_{from,to}

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Message-ID: <20200905071447.GA13228@lst.de>
Date:   Sat, 5 Sep 2020 09:14:47 +0200
From:   Christoph Hellwig <hch@....de>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Christoph Hellwig <hch@....de>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 3/8] asm-generic: fix unaligned access hamdling in
 raw_copy_{from,to}_user

On Fri, Sep 04, 2020 at 08:04:34PM +0200, Arnd Bergmann wrote:
> >         if (__builtin_constant_p(n)) {
> >                 switch(n) {
> >                 case 1:
> > -                       *(u8 *)to = *(u8 __force *)from;
> > +                       *(u8 *)to = get_unaligned((u8 __force *)from);
> >                         return 0;
> >                 case 2:
> > -                       *(u16 *)to = *(u16 __force *)from;
> > +                       *(u16 *)to = get_unaligned((u16 __force *)from);
> >                         return 0;
> 
> The change look correct and necessary, but I wonder if this could be done
> in a way that is a little easier on the compiler than the nested switch/case.
> 
> If I see it right, __put_user() and __get_user() can probably
> be reduced to a plain put_unaligned() and get_unaligned() here,
> which would simplify these a lot.
> 
> In turn it seems that the generic raw_copy_to_user() can just be the
> a plain memcpy(), IIRC the optimization for small sizes should also
> be done by modern compilers whenever they can.

Sure, I can look into that.