linux-kernel - Re: [PATCH V2] LoongArch: Add unaligned access support

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Message-ID: <CAAhV-H5WM97+Um9ULjv7gT3i5pJ3GfQ8ex6NRNr6uzmMgdVY_Q@mail.gmail.com>
Date:   Tue, 18 Oct 2022 10:24:36 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Huacai Chen <chenhuacai@...ngson.cn>,
        "loongarch@...ts.linux.dev" <loongarch@...ts.linux.dev>,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] LoongArch: Add unaligned access support

Hi, David,

On Mon, Oct 17, 2022 at 8:58 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Huacai Chen
> > Sent: 17 October 2022 03:24
> >
> > Loongson-2 series (Loongson-2K500, Loongson-2K1000) don't support
> > unaligned access in hardware, while Loongson-3 series (Loongson-3A5000,
> > Loongson-3C5000) are configurable whether support unaligned access in
> > hardware. This patch add unaligned access emulation for those LoongArch
> > processors without hardware support.
> >
> ...
> > +     /*
> > +      * This load never faults.
> > +      */
> > +     __get_inst(&insn.word, pc, user);
>
> On what basis does it never fault?
> Any user access can fault.
> If nothing else another thread of the process can unmap
> the page.
Yes, this can happen, since __get_inst() handles fault, we can just
remove the comment.

>
> > +     if (user && !access_ok(addr, 8))
> > +             goto sigbus;
>
> Surely that is technically wrong - a two or four byte
> access is valid right at the end of valid user addreeses.
Yes, this check should be moved to each case.

>
> > +
> > +     if (insn.reg2i12_format.opcode == ldd_op ||
> > +             insn.reg2i14_format.opcode == ldptrd_op ||
> > +             insn.reg3_format.opcode == ldxd_op) {
> > +             res = unaligned_read(addr, &value, 8, 1);
>
> That is the most horrid indentation of long lines I've
> ever seen.
> I'd also guess you can common up some of this code
> by looking at the instruction field that include the
> transfer width.
>
> The long elsif list will generate horrid code.
> But maybe since you've just taken a fault it really
> doesn't matter.
> Indeed just emulating in C using byte accesses
> it probably fine.
I want to keep the assembly, because we can use more efficient methods
with the upcoming alternative mechanism.

Huacai
>
>         David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>