linux-kernel - Re: [PATCH] sh: Implement __get_user_u64() required for 64-bit get

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Message-ID: <20200601030300.GT1079@brightrain.aerifal.cx>
Date:   Sun, 31 May 2020 23:03:01 -0400
From:   Rich Felker <dalias@...c.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Linux-sh list <linux-sh@...r.kernel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Michael Karcher <kernel@...rcher.dialup.fu-berlin.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sh: Implement __get_user_u64() required for 64-bit
 get_user()

On Sun, May 31, 2020 at 12:43:11PM +0200, Geert Uytterhoeven wrote:
> Hi Adrian,
> 
> On Sun, May 31, 2020 at 11:59 AM John Paul Adrian Glaubitz
> <glaubitz@...sik.fu-berlin.de> wrote:
> > On 5/31/20 11:54 AM, John Paul Adrian Glaubitz wrote:
> > > On 5/31/20 11:52 AM, Geert Uytterhoeven wrote:
> > >> As this is the 64-bit variant, I think this single move should be
> > >> replaced by a double move:
> > >>
> > >>        "mov  #0,%R1\n\t" \
> > >>        "mov  #0,%S1\n\t" \
> > >>
> > >> Same for the big endian version below.
> > >>
> > >> Disclaimer: uncompiled, untested, no SH assembler expert.
> > >
> > > Right, this makes sense. I'll send a new patch shortly.
> >
> > Hmm, this change is not the case for __put_user_asm() vs. __put_user_u64().
> > But I have to admit, I don't know what the part below "3:\n\t" is for.
> 
> It's part of the exception handling, in case the passed (userspace) pointer
> points to an inaccessible address, and triggers an exception.
> 
> For an invalid store, nothing is done, besides returning -EFAULT.
> Hence there's no "mov #0, %1\n\t" in the put_user case.
> For an invalid load, the data is replaced by zero, and -EFAULT is returned.
> 
> > +__asm__ __volatile__( \
> > +       "1:\n\t" \
> > +       "mov.l  %2,%R1\n\t" \
> > +       "mov.l  %T2,%S1\n\t" \
> > +       "2:\n" \
> 
> (reordering the two sections for easier explanation)
> 
> > +       ".section       __ex_table,\"a\"\n\t" \
> > +       ".long  1b, 3b\n\t" \
> 
> In case an exception happens for the instruction at 1b, jump to 3b.
> 
> Note that the m68k version has two entries here: one for each half of
> the 64-bit access[*].
> I don't know if that is really needed (and thus SH needs it, too), or if
> the exception code handles subsequent instructions automatically.

Can I propose a different solution? For archs where there isn't
actually any 64-bit load or store instruction, does it make sense to
be writing asm just to do two 32-bit loads/stores, especially when
this code is not in a hot path?

What about just having the 64-bit versions call the corresponding
32-bit version twice? (Ideally this would even be arch-generic and
could replace the m68k asm.) It would return EFAULT if either of the
32-bit calls did.

Rich