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

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Date:   Mon, 1 Jun 2020 12:57:00 -0400
From:   Rich Felker <dalias@...c.org>
To:     John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        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 Mon, Jun 01, 2020 at 11:13:26AM +0200, John Paul Adrian Glaubitz wrote:
> Hello!
> 
> On 6/1/20 11:02 AM, Geert Uytterhoeven wrote:
> >> 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.
> > 
> > Yes, that's an option, too.
> 
> That's the solution that Michael Karcher suggested to me as an alternative
> when I talked to him off-list.
> 
> While I understand that it works, I don't like the inconsistency and I also
> don't see why we should opt for a potentially slower solution when we can
> used the fastest one.
> 
> I'm also not sure how the exception handling would properly work when you
> have two invocations of __get_user_asm().
> 
> My current approach is consistent with the existing code, so I think it's
> the natural choice. I just need someone with more experience in SH assembler
> than me that the solution is correct.
> 
> I have already pinged Niibe-san in private, he'll hopefully get back to me
> within the next days.

I don't have an objection to doing it the way you've proposed, but I
don't think there's any performance distinction or issue with the two
invocations.