linux-kernel - Re: [RFC PATCH V3 00/11] riscv: Add vector ISA support

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Date:   Thu, 12 Mar 2020 11:14:29 +0800
From:   LIU Zhiwei <zhiwei_liu@...ky.com>
To:     Greentime Hu <greentime.hu@...ive.com>
Cc:     guoren@...nel.org, Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>, Anup.Patel@....com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, arnd@...db.de,
        linux-csky@...r.kernel.org,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Dave Martin <Dave.Martin@....com>, Alistair.Francis@....com,
        wenmeng_zhang <wenmeng_zhang@...ky.com>
Subject: Re: [RFC PATCH V3 00/11] riscv: Add vector ISA support



On 2020/3/10 17:19, Greentime Hu wrote:
> On Tue, Mar 10, 2020 at 4:54 PM Greentime Hu <greentime.hu@...ive.com> wrote:
>> On Mon, Mar 9, 2020 at 6:27 PM LIU Zhiwei <zhiwei_liu@...ky.com> wrote:
>>> On 2020/3/9 11:41, Greentime Hu wrote:
>>>> On Sun, Mar 8, 2020 at 5:50 PM <guoren@...nel.org> wrote:
>>>>> From: Guo Ren <guoren@...ux.alibaba.com>
>>>>>
>>>>> The implementation follow the RISC-V "V" Vector Extension draft v0.8 with
>>>>> 128bit-vlen and it's based on linux-5.6-rc3 and tested with qemu [1].
>>>>>
>>>>> The patch implement basic context switch, sigcontext save/restore and
>>>>> ptrace interface with a new regset NT_RISCV_VECTOR. Only fixed 128bit-vlen
>>>>> is implemented. We need to discuss about vlen-size for libc sigcontext and
>>>>> ptrace (the maximum size of vlen is unlimited in spec).
>>>>>
>>>>> Puzzle:
>>>>> Dave Martin has talked "Growing CPU register state without breaking ABI" [2]
>>>>> before, and riscv also met vlen size problem. Let's discuss the common issue
>>>>> for all architectures and we need a better solution for unlimited vlen.
>>>>>
>>>>> Any help are welcomed :)
>>>>>
>>>>>    1: https://github.com/romanheros/qemu.git branch:vector-upstream-v3
>>>> Hi Guo,
>>>>
>>>> Thanks for your patch.
>>>> It seems the qemu repo doesn't have this branch?
>>> Hi Greentime,
>>>
>>> It's a promise from me. Now it's ready.  You can turn on vector by
>>> "qemu-system-riscv64 -cpu rv64,v=true,vext_spec=v0.7.1".
>>>
>>> Zhiwei
>>>
>>>
>> Hi Zhiwei,
>>
>> Thank you, I see the branch in the repo now. I will give it a try and
>> let you know if I have any problem. :)
> Hi Zhiwei & Guo,
>
> It seems current version only support v0.7.1 in qemu but this patchset
> is verified in qemu too and it is based on 0.8.
> Would you please provide the qemu with 0.8 vector spec supported?
Hi Greentime,
vector-upstream-v3 only supports v0.7.1. It  is under reviewed in QEMU 
community.
Maybe I will also support v0.8 after it is merged.

As Guo Ren said, the kernel patch set works both  for v0.7.1 and v0.8,
which only uses the common instructions and CSRs.
> or
> Did I miss something?
>
> 489             if (cpu->cfg.vext_spec) {
> 490                 if (!g_strcmp0(cpu->cfg.vext_spec, "v0.7.1")) {
> 491                     vext_version = VEXT_VERSION_0_07_1;
> 492                 } else {
> 493                     error_setg(errp,
> 494                            "Unsupported vector spec version '%s'",
> 495                            cpu->cfg.vext_spec);
> 496                     return;
> 497                 }
> 498             }
>
> By the way, can I specify vlen in Qemu?
Yes, you can specify vlen through QEMU command line like
“-cpu rv64,v=true,vext_spec=v0.7.1,vlen=256”

Currently , vlen supports up to 512 bits, with a default value 128 bits.

> Thank you. :)