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

[Jessica Clarke <jrtc27@jrtc27.com>]

On 7 Jan 2022, at 13:28, Greentime Hu <greentime.hu@sifive.com> wrote:
> 
> Palmer Dabbelt <palmer@dabbelt.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@sifive.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@sifive.com>
>>> Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
>>> Signed-off-by: Greentime Hu <greentime.hu@sifive.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.

That struct has an alignment of 4 bytes. It doesn’t make sense to put
the padding there; it should be wherever the 16 byte alignment is
introduced, which looks like your __sc_riscv_v_state below (assuming
you need to explicitly name the padding and can’t just rely on implicit
compiler padding; presumably you need it so you can guarantee it’s zero
when written to userspace memory?).

Especially since the amount of padding you need in __riscv_v_state if
doing it this way depends on the size of __riscv_ctx_hdr, because that
happens to be in __sc_riscv_v_state. This is quite fragile and
non-obvious as it stands.

Jess

> 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);
> 
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