Messages in this thread | | | From | Huacai Chen <> | Date | Tue, 18 Oct 2022 15:32:50 +0800 | Subject | Re: [PATCH V2] LoongArch: Add unaligned access support |
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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/ >
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