[RFC PATCH 4/4] lib: sbi: add support for the Smdbltrp extension

[Anup Patel]

On Thu, Apr 18, 2024 at 7:25 PM Clément Léger <cleger at rivosinc.com> wrote:
>
> Add support for the Smdbltrp[1]. Fuirst thing to do is clearing MDT on
> entry after setting the first MTVEC (since MDT is reste to 1).
> Additionnaly, during trap handling, clear MDT once all critical CSRs
> have been saved and in return path, restore MSTATUS before restoring
> MEPC to avoid taking another trap which would clobber it.
>
> Link: https://github.com/riscv/riscv-double-trap/releases/download/v0.56/riscv-double-trap.pdf [1]
> Signed-off-by: Clément Léger <cleger at rivosinc.com>
> ---
>  firmware/fw_base.S           | 29 ++++++++++++++++++++++++++---
>  include/sbi/riscv_encoding.h |  2 ++
>  2 files changed, 28 insertions(+), 3 deletions(-)
>
> diff --git a/firmware/fw_base.S b/firmware/fw_base.S
> index 9f995a2..cd71ef4 100644
> --- a/firmware/fw_base.S
> +++ b/firmware/fw_base.S
> @@ -31,6 +31,16 @@
>         add     \__d4, \__s4, zero
>  .endm
>
> +.macro CLEAR_MDT tmp, have_mstatush
> +       .if \have_mstatush
> +       li      \tmp, MSTATUSH_MDT
> +       csrc    CSR_MSTATUSH, \tmp
> +       .else
> +       li      \tmp, MSTATUS_MDT
> +       csrc    CSR_MSTATUS, \tmp
> +       .endif
> +.endm
> +
>         .section .entry, "ax", %progbits
>         .align 3
>         .globl _start
> @@ -89,6 +99,13 @@ _bss_zero:
>         lla     s4, _start_hang
>         csrw    CSR_MTVEC, s4
>
> +       /* We are now ready to take a trap, clear MDT */
> +#if __riscv_xlen > 32
> +       CLEAR_MDT t0, 0
> +#else
> +       CLEAR_MDT t0, 1
> +#endif
> +

This should be done just before calling sbi_init() so that
we clear MDT even when the CPU is power-down and
power-up at runtime.

>         /* Setup temporary stack */
>         lla     s4, _fw_end
>         li      s5, (SBI_SCRATCH_SIZE * 2)
> @@ -555,6 +572,9 @@ memcmp:
>         li      t0, 0
>  .endif
>         REG_S   t0, (SBI_TRAP_REGS_SIZE + SBI_TRAP_INFO_OFFSET(gva))(sp)
> +
> +       /* We are ready to take another trap, clear MDT */
> +       CLEAR_MDT t0, \have_mstatush

Better to do this in TRAP_CALL_C_ROUTINE() before entering
sbi_trap_handler().

>  .endm
>
>  .macro TRAP_CALL_C_ROUTINE
> @@ -597,15 +617,18 @@ memcmp:
>  .endm
>
>  .macro TRAP_RESTORE_MEPC_MSTATUS have_mstatush
> -       /* Restore MEPC and MSTATUS CSRs */
> -       REG_L   t0, SBI_TRAP_REGS_OFFSET(mepc)(a0)
> -       csrw    CSR_MEPC, t0
> +       /*
> +        * Restore MSTATUS and MEPC CSRs starting with MSTATUS to set MDT
> +        * flags since we can not take a trap now or MEPC would be cloberred

s/cloberred/clobbered/

> +        */
>         REG_L   t0, SBI_TRAP_REGS_OFFSET(mstatus)(a0)
>         csrw    CSR_MSTATUS, t0
>         .if \have_mstatush
>         REG_L   t0, SBI_TRAP_REGS_OFFSET(mstatusH)(a0)
>         csrw    CSR_MSTATUSH, t0
>         .endif
> +       REG_L   t0, SBI_TRAP_REGS_OFFSET(mepc)(a0)
> +       csrw    CSR_MEPC, t0
>  .endm
>
>  .macro TRAP_RESTORE_A0_T0
> diff --git a/include/sbi/riscv_encoding.h b/include/sbi/riscv_encoding.h
> index 8628e5a..eed28f2 100644
> --- a/include/sbi/riscv_encoding.h
> +++ b/include/sbi/riscv_encoding.h
> @@ -42,12 +42,14 @@
>  #define MSTATUS_GVA                    _ULL(0x0000004000000000)
>  #define MSTATUS_GVA_SHIFT              38
>  #define MSTATUS_MPV                    _ULL(0x0000008000000000)
> +#define MSTATUS_MDT                    _ULL(0x0000200000000000)
>  #else
>  #define MSTATUSH_SBE                   _UL(0x00000010)
>  #define MSTATUSH_MBE                   _UL(0x00000020)
>  #define MSTATUSH_GVA                   _UL(0x00000040)
>  #define MSTATUSH_GVA_SHIFT             6
>  #define MSTATUSH_MPV                   _UL(0x00000080)
> +#define MSTATUSH_MDT                   _UL(0x00002000)
>  #endif
>  #define MSTATUS32_SD                   _UL(0x80000000)
>  #define MSTATUS64_SD                   _ULL(0x8000000000000000)
> --
> 2.43.0
>
>
> --
> opensbi mailing list
> opensbi at lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/opensbi

Regards,
Anup