linux-kernel - Re: [PATCH v2 3/3] RISC-V: add T-Head vector errata handling

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Message-ID: <20230624-844c5c4701e8acf9bcf150aa@orel>
Date:   Sat, 24 Jun 2023 12:59:41 +0200
From:   Andrew Jones <ajones@...tanamicro.com>
To:     Stefan O'Rear <sorear@...tmail.com>
Cc:     Heiko Stuebner <heiko@...ech.de>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Palmer Dabbelt <palmer@...belt.com>, paul.walmsley@...ive.com,
        linux-riscv@...ts.infradead.org, samuel@...lland.org,
        guoren@...nel.org, christoph.muellner@...ll.eu,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] RISC-V: add T-Head vector errata handling

On Sat, Jun 24, 2023 at 01:18:26AM -0400, Stefan O'Rear wrote:
> On Fri, Jun 23, 2023, at 6:40 AM, Heiko Stübner wrote:
...
> >> > +
> >> > +/*
> >> > + * Vector 0.7.1 as used for example on T-Head Xuantie cores, uses an older
> >> > + * encoding for vsetvli (ta, ma vs. d1), so provide an instruction for
> >> > + * vsetvli	t4, x0, e8, m8, d1
> >> > + */
> >> > +#define THEAD_VSETVLI_T4X0E8M8D1	".long	0x00307ed7\n\t"
> >> > +
> >> > +/*
> >> > + * While in theory, the vector-0.7.1 vsb.v and vlb.v result in the same
> >> > + * encoding as the standard vse8.v and vle8.v, compilers seem to optimize
> >> > + * the call resulting in a different encoding and then using a value for
> >> > + * the "mop" field that is not part of vector-0.7.1
> >> > + * So encode specific variants for vstate_save and _restore.
> >> > + */
> >> > +#define THEAD_VSB_V_V0T0		".long	0x02028027\n\t"
> >> > +#define THEAD_VSB_V_V8T0		".long	0x02028427\n\t"
> >> > +#define THEAD_VSB_V_V16T0		".long	0x02028827\n\t"
> >> > +#define THEAD_VSB_V_V24T0		".long	0x02028c27\n\t"
> >> > +#define THEAD_VLB_V_V0T0		".long	0x012028007\n\t"
> >> > +#define THEAD_VLB_V_V8T0		".long	0x012028407\n\t"
> >> > +#define THEAD_VLB_V_V16T0		".long	0x012028807\n\t"
> >> > +#define THEAD_VLB_V_V24T0		".long	0x012028c07\n\t"
> 
> .insn isn't supported by the kernel's minimum binutils version, but it _is_
> supported by the oldest version of binutils that can assemble rvv 1.0
> instructions.  OP_V requires 2.39 so I use a literal 0x57 instead.
> 
> very untested, and I leave it to your judgement whether it actually improves
> readability:
> 
> #define THEAD_VSETVLI_T4X0E8M8D1  ".insn i 0x57, 7, t4, x0, 3\n\t"
> #define THEAD_VSB_V_V0T0		".insn r STORE_FP, 0, 1, x0,  t0, x0\n\t"
> #define THEAD_VSB_V_V8T0		".insn r STORE_FP, 0, 1, x8,  t0, x0\n\t"
> #define THEAD_VSB_V_V16T0		".insn r STORE_FP, 0, 1, x16, t0, x0\n\t"
> #define THEAD_VSB_V_V24T0		".insn r STORE_FP, 0, 1, x24, t0, x0\n\t"
> #define THEAD_VSB_V_V0T0		".insn r LOAD_FP,  0, 9, x0,  t0, x0\n\t"
> #define THEAD_VSB_V_V8T0		".insn r LOAD_FP,  0, 9, x8,  t0, x0\n\t"
> #define THEAD_VSB_V_V16T0		".insn r LOAD_FP,  0, 9, x16, t0, x0\n\t"
> #define THEAD_VSB_V_V24T0		".insn r LOAD_FP,  0, 9, x24, t0, x0\n\t"
>

We have the INSN_R() macro in arch/riscv/include/asm/insn-def.h for stuff
like this.

Thanks,
drew