Messages in this thread | | | Date | Tue, 18 Oct 2022 11:29:39 +0800 | Subject | Re: [PATCH V2] LoongArch: Add unaligned access support | From | WANG Xuerui <> |
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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.
-- WANG "xen0n" Xuerui
Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/
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