linux-kernel - Re: [PATCH v9 08/17] riscv: Add vector struct and assembler definitions

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Message-ID: <CAHCEehJhMQf9jtVkTvorriv6xY5QnEDmN_JjM-UxcCj9UL86Lg@mail.gmail.com>
Date:   Fri, 7 Jan 2022 21:28:50 +0800
From:   Greentime Hu <greentime.hu@...ive.com>
To:     Palmer Dabbelt <palmer@...belt.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Albert Ou <aou@...s.berkeley.edu>
Subject: Re: [PATCH v9 08/17] riscv: Add vector struct and assembler definitions

Palmer Dabbelt <palmer@...belt.com> 於 2021年12月15日 週三 上午12:29寫道：
>
> On Tue, 09 Nov 2021 01:48:20 PST (-0800), greentime.hu@...ive.com wrote:
> > Add vector state context struct in struct thread and asm-offsets.c
> > definitions.
> >
> > The vector registers will be saved in datap pointer of __riscv_v_state. It
> > will be dynamically allocated in kernel space. It will be put right after
> > the __riscv_v_state data structure in user space.
> >
> > Co-developed-by: Vincent Chen <vincent.chen@...ive.com>
> > Signed-off-by: Vincent Chen <vincent.chen@...ive.com>
> > Signed-off-by: Greentime Hu <greentime.hu@...ive.com>
> > ---
> >  arch/riscv/include/asm/processor.h   |  1 +
> >  arch/riscv/include/uapi/asm/ptrace.h | 11 +++++++++++
> >  arch/riscv/kernel/asm-offsets.c      |  6 ++++++
> >  3 files changed, 18 insertions(+)
> >
> > diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> > index 46b492c78cbb..a268f1382e52 100644
> > --- a/arch/riscv/include/asm/processor.h
> > +++ b/arch/riscv/include/asm/processor.h
> > @@ -35,6 +35,7 @@ struct thread_struct {
> >       unsigned long s[12];    /* s[0]: frame pointer */
> >       struct __riscv_d_ext_state fstate;
> >       unsigned long bad_cause;
> > +     struct __riscv_v_state vstate;
> >  };
> >
> >  /* Whitelist the fstate from the task_struct for hardened usercopy */
> > diff --git a/arch/riscv/include/uapi/asm/ptrace.h b/arch/riscv/include/uapi/asm/ptrace.h
> > index 882547f6bd5c..bd3b8a710246 100644
> > --- a/arch/riscv/include/uapi/asm/ptrace.h
> > +++ b/arch/riscv/include/uapi/asm/ptrace.h
> > @@ -77,6 +77,17 @@ union __riscv_fp_state {
> >       struct __riscv_q_ext_state q;
> >  };
> >
> > +struct __riscv_v_state {
> > +     unsigned long vstart;
> > +     unsigned long vl;
> > +     unsigned long vtype;
> > +     unsigned long vcsr;
>
> Don't we also need vlen to adequately determine the vector state?
> Otherwise we're going to end up dropping some state when vl isn't vlmax,
> which IIUC isn't legal.

Do you mean vlenb? Since it is a constant value, we don't need to
save/restore it in the context.

> > +     void *datap;
> > +#if __riscv_xlen == 32
> > +     __u32 __padding;
> > +#endif
>
> Why is there padding?

To keep vector registers saved in a 16-bytes aligned address for rv32.

struct __riscv_ctx_hdr {
        __u32 magic;
        __u32 size;
};
struct __sc_riscv_v_state {
        struct __riscv_ctx_hdr head;
        struct __riscv_v_state v_state;
} __attribute__((aligned(16)));

rv64 => 48bytes -> 16byte aligned
rv32 => 32bytes -> 16byte aligned

This struct and vector registers will be copied to
sigcontext.reserved[] for signal handler so we'd like to keep it is
16-byte aligned.

struct sigcontext {
        struct user_regs_struct sc_regs;
        union __riscv_fp_state sc_fpregs;
        /*
         * 4K + 128 reserved for vector state and future expansion.
         * This space is enough to store the vector context whose VLENB
         * is less or equal to 128.
         * (The size of the vector context is 4144 byte as VLENB is 128)
         */
        __u8 __reserved[4224] __attribute__((__aligned__(16)));
};


> > +};
> > +
> >  #endif /* __ASSEMBLY__ */
> >
> >  #endif /* _UAPI_ASM_RISCV_PTRACE_H */
> > diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
> > index 90f8ce64fa6f..34f43c84723a 100644
> > --- a/arch/riscv/kernel/asm-offsets.c
> > +++ b/arch/riscv/kernel/asm-offsets.c
> > @@ -72,6 +72,12 @@ void asm_offsets(void)
> >       OFFSET(TSK_STACK_CANARY, task_struct, stack_canary);
> >  #endif
> >
> > +     OFFSET(RISCV_V_STATE_VSTART, __riscv_v_state, vstart);
> > +     OFFSET(RISCV_V_STATE_VL, __riscv_v_state, vl);
> > +     OFFSET(RISCV_V_STATE_VTYPE, __riscv_v_state, vtype);
> > +     OFFSET(RISCV_V_STATE_VCSR, __riscv_v_state, vcsr);
> > +     OFFSET(RISCV_V_STATE_DATAP, __riscv_v_state, datap);
> > +
> >       DEFINE(PT_SIZE, sizeof(struct pt_regs));
> >       OFFSET(PT_EPC, pt_regs, epc);
> >       OFFSET(PT_RA, pt_regs, ra);