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

[Greentime Hu <greentime.hu@sifive.com>]

<guoren@kernel.org> 於 2020年3月8日 週日 下午5:50寫道：
>
> From: Guo Ren <guoren@linux.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
>  2: https://blog.linuxplumbersconf.org/2017/ocw/sessions/4671.html
>

Hi Ren,

Thanks for the patch. I have some ideas about the vlen and sigcontext.
Since vlen may not be fixed of each RISC-V cores and it could be super
big, it means we have to allocate the memory dynamically.
In kernel space, we may use a pointer in the context data structure.
Something like https://github.com/torvalds/linux/blob/master/arch/arm64/kernel/fpsimd.c#L498
In user space, we need to let user space know the length of vector
registers. We may create a special header in sigcontext. Something
like https://github.com/torvalds/linux/blob/master/arch/arm64/include/uapi/asm/sigcontext.h#L36
https://github.com/torvalds/linux/blob/master/arch/arm64/include/uapi/asm/sigcontext.h#L127

For the implementation in makecontext, swapcontext, getcontext,
setcontext of glibc, we may not need to port because it seems to be
deprecated?
https://stackoverflow.com/questions/4298986/is-there-something-to-replace-the-ucontext-h-functions

For the unwinding implementation of libgcc since it needs to know the
meaning of data structure is  changed. It also need to be port.

> ---
> Changelog V3
>  - Rebase linux-5.6-rc3 and tested with qemu
>  - Seperate patches with Anup's advice
>  - Give out a ABI puzzle with unlimited vlen
>
> Changelog V2
>  - Fixup typo "vecotr, fstate_save->vstate_save".
>  - Fixup wrong saved registers' length in vector.S.
>  - Seperate unrelated patches from this one.
>
> Guo Ren (11):
>   riscv: Separate patch for cflags and aflags
>   riscv: Rename __switch_to_aux -> fpu
>   riscv: Extending cpufeature.c to detect V-extension
>   riscv: Add CSR defines related to VECTOR extension
>   riscv: Add vector feature to compile
>   riscv: Add has_vector detect
>   riscv: Reset vector register
>   riscv: Add vector struct and assembler definitions
>   riscv: Add task switch support for VECTOR
>   riscv: Add ptrace support
>   riscv: Add sigcontext save/restore
>
>  arch/riscv/Kconfig                       |   9 ++
>  arch/riscv/Makefile                      |  19 ++-
>  arch/riscv/include/asm/csr.h             |  17 ++-
>  arch/riscv/include/asm/processor.h       |   1 +
>  arch/riscv/include/asm/switch_to.h       |  54 ++++++-
>  arch/riscv/include/uapi/asm/elf.h        |   1 +
>  arch/riscv/include/uapi/asm/hwcap.h      |   1 +
>  arch/riscv/include/uapi/asm/ptrace.h     |   9 ++
>  arch/riscv/include/uapi/asm/sigcontext.h |   1 +
>  arch/riscv/kernel/Makefile               |   1 +
>  arch/riscv/kernel/asm-offsets.c          | 187 +++++++++++++++++++++++
>  arch/riscv/kernel/cpufeature.c           |  12 +-
>  arch/riscv/kernel/entry.S                |   2 +-
>  arch/riscv/kernel/head.S                 |  49 +++++-
>  arch/riscv/kernel/process.c              |  10 ++
>  arch/riscv/kernel/ptrace.c               |  41 +++++
>  arch/riscv/kernel/signal.c               |  40 +++++
>  arch/riscv/kernel/vector.S               |  84 ++++++++++
>  include/uapi/linux/elf.h                 |   1 +
>  19 files changed, 524 insertions(+), 15 deletions(-)
>  create mode 100644 arch/riscv/kernel/vector.S
>
> --
> 2.17.0
>