Re: [PATCH V2] LoongArch: Add unaligned access support

From: Huacai Chen <chenhuacai@kernel.org>
To: WANG Xuerui <kernel@xen0n.name>
Cc: David Laight <David.Laight@aculab.com>,
	Huacai Chen <chenhuacai@loongson.cn>,
	"loongarch@lists.linux.dev" <loongarch@lists.linux.dev>,
	Xuefeng Li <lixuefeng@loongson.cn>,
	Tiezhu Yang <yangtiezhu@loongson.cn>, Guo Ren <guoren@kernel.org>,
	Jiaxun Yang <jiaxun.yang@flygoat.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH V2] LoongArch: Add unaligned access support
Date: Tue, 18 Oct 2022 15:32:50 +0800	[thread overview]
Message-ID: <CAAhV-H5y5wz0VzFEHrkDpap4wczWXBn_ibL3LW_JQyYkMzjDXQ@mail.gmail.com> (raw)
In-Reply-To: <c34a277a-1735-65e6-c97d-fcc2ac3d57e6@xen0n.name>

On Tue, Oct 18, 2022 at 11:29 AM WANG Xuerui <kernel@xen0n.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@aculab.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/
>