[PATCH 0/9] Improve trap handling for nested traps

[Bo Gan]

On 3/11/24 8:43 PM, Anup Patel wrote:
> On Tue, Mar 12, 2024 at 6:31 AM Bo Gan <ganboing at gmail.com> wrote:
>>
>> On 3/11/24 9:09 AM, Anup Patel wrote:
>>> Nested traps will be a common when dealing with RAS error traps so
>>> this series improves trap handling for nested traps by introducing
>>> a linked-list based trap context chain.
>>>
>>> These patches can also be found the trap_handling_imp_v1 branch at
>>> https://github.com/avpatel/opensbi.git
>>>
>>> Anup Patel (9):
>>>     lib: sbi: Remove sbi_trap_exit() and related code
>>>     include: sbi: Add trap_context pointer in struct sbi_scratch
>>>     lib: sbi: Introduce trap context
>>>     lib: sbi: Simplify parameters of misaligned and access fault handlers
>>>     lib: sbi: Simplify parameters of sbi_illegal_insn_handler()
>>>     lib: sbi: Remove regs paramter of sbi_irqchip_process()
>>>     lib: sbi: Remove regs parameter from trap irq handling functions
>>>     lib: sbi: Pass trap context pointer to sbi_ecall_handler()
>>>     lib: sbi: Extend sbi_trap_error() to dump state in a nested trap
>>>
>>>    firmware/fw_base.S             |  14 +--
>>>    include/sbi/sbi_ecall.h        |   4 +-
>>>    include/sbi/sbi_illegal_insn.h |   4 +-
>>>    include/sbi/sbi_irqchip.h      |   5 +-
>>>    include/sbi/sbi_scratch.h      |  14 +--
>>>    include/sbi/sbi_trap.h         |  24 ++++-
>>>    include/sbi/sbi_trap_ldst.h    |  12 +--
>>>    lib/sbi/sbi_ecall.c            |   3 +-
>>>    lib/sbi/sbi_illegal_insn.c     |  14 +--
>>>    lib/sbi/sbi_irqchip.c          |  10 +-
>>>    lib/sbi/sbi_trap.c             | 186 +++++++++++++++++----------------
>>>    lib/sbi/sbi_trap_ldst.c        |  67 ++++++------
>>>    lib/utils/irqchip/imsic.c      |   2 +-
>>>    13 files changed, 185 insertions(+), 174 deletions(-)
>>>
>>
>> Hi Anup,
>>
>> Can you help providing an example for nested traps and the RAS extension?
>> The closest spec I can find (https://github.com/riscv/riscv-ssrastraps) is
>> still empty. I'm wondering where I can find the related documentation.
> 
> Refer, "3.1.15. Machine Cause Register" of the draft Priv v1.13 specification.
> https://github.com/riscv/riscv-isa-manual/releases/download/riscv-isa-release-157641b-2024-03-12/priv-isa-asciidoc.pdf
> 
> Regards,
> Anup
> 


Hi Anup, Thanks for the pointer. I know what nested trap is. My concern is when do
we expect such trap to happen? Does it mean for every memory access in M mode, it
might trigger a RAS fault, causing a nested trap? Or even the fault can be delivered
asynchronously? If that's the case, how can we even safely handle nested traps? E.g.,
The entry of _trap_handler doesn't look like reentrant-safe to me. Perhaps you plan
to enhance it in later patches?

Bo