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

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Date:   Tue, 18 Oct 2022 15:32:50 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     WANG Xuerui <kernel@...0n.name>
Cc:     David Laight <David.Laight@...lab.com>,
        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>,
        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

On Tue, Oct 18, 2022 at 11:29 AM WANG Xuerui <kernel@...0n.name> wrote:
>
> On 2022/10/18 10:24, Huacai Chen wrote:
> > 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.
>
> What about my more structured approach in another reply that avoids the
> huge else-if conditions? Both the terrible line wraps and codegen could
> be avoided.
OK, let me try.

Huacai
>
> --
> WANG "xen0n" Xuerui
>
> Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/
>