On Tue, 19 Jan 2021 10:04:28 -0800, Earl Killian <earl.killian@...> said:
| I would like a clarification on whether this replaces the existing NMI, or are you saying there are two different things, NMI and RNMI? I doubt it, but I wanted to check.
Enhances NMI by adding state to make resumable.
| I am concerned that the rnmie is hidden and only settable by MNRET. This means that to re-enable NMI for a portion of a NMI handler (e.g. after saving all the appropriate information to a NMI stack) one must write mnepc and then MNRET there, but there is no way to disable it for the real MNRET.
Should have been visible but only settable by M-mode software.
Krste
|| On Jan 18, 2021, at 18:39, Krste Asanovic <krste@...> wrote:
||
||
|| Current RISC-V specs only have a non-resumable NMI definition. The
|| following proposal would add resumable NMI support. This was one of
|| the features requested for priv 1.12 or RVA/RVM22.
||
|| This is up for discussion, but I think it is small enough to go
|| through fast track process.
||
|| Krste
||
|| :sectnums:
|| :toc: left
||
|| = Resumable NMI support in RISC-V
|| Version 0.2.1-Draft
||
|| == Background and Motivation
||
|| The RISC-V privileged architecture version 1.11 supports only
|| unresumable non-maskable interrupts (UNMIs), where the NMI jumps to a
|| handler in machine mode, overwriting the current `mepc` and `mcause`
|| register values. If the hart had been executing machine-mode code in
|| a trap handler, the previous values in `mepc` and `mcause` would not
|| be recoverable and so execution is not generally resumable.
||
|| This proposal adds support for resumable non-maskable interrupts
|| (RNMIs) to RISC-V. The extension adds four new CSRs (`mnepc`,
|| `mncause`, `mnstatus`, and `mnscratch`) to hold the interrupted state,
|| and a new instruction to resume from the RNMI handler.
||
|| == RNMI Interrupt Signals
||
|| The `rnmi` interrupt signals are inputs to
|| the hart. These interrupts have higher priority than any other
|| interrupt or exception on the hart and cannot be disabled by software.
|| Specifically, they are not disabled by clearing the `mstatus.mie`
|| register.
||
|| == RNMI Handler Addresses
||
|| The RNMI interrupt trap handler address is implementation-defined.
||
|| RNMI also has an associated exception trap handler address, which is
|| implementation defined.
||
|| == New RNMI CSRs
||
|| This proposal adds additional M-mode CSRs to enable a resumable
|| non-maskable interrupt (RNMI).
||
|| .NMI additional CSRs
|| [cols="2,2,2,2"]
|| [%autowidth]
|| |===
|| | Number | Privilege | Name | Description
||
|| | 0x350 | MRW | `mnscratch` | Resumable Non-maskable scratch register
|| | 0x351 | MRW | `mnepc` | Resumable Non-maskable EPC value
|| | 0x352 | MRW | `mncause` | Resumable Non-maskable cause value
|| | 0x353 | MRW | `mnstatus` | Resumable Non-maskable status
|| |===
||
|| The `mnscratch` CSR holds an XLEN-bit read-write register which
|| enables the NMI trap handler to save and restore the context that was
|| interrupted.
||
|| The `mnepc` CSR is an XLEN-bit read-write register which on entry
|| to the NMI trap handler holds the PC of the instruction that took the
|| interrupt. The lowest bit of `mnepc` is hardwired to zero.
||
|| The `mncause` CSR holds the reason for the NMI, with bit XLEN-1 set to
|| 1, and the NMI cause encoded in the least-significant bits or zero if
|| NMI causes are not supported.
||
|| The `mnstatus` CSR holds a two-bit field which on entry to the trap
|| handler holds the privilege mode of the interrupted context encoded in
|| bits `mnstatus[12:11]` in the same manner as `mstatus.mpp`. The other
|| bits in `mnstatus` are _reserved_, but software should write zeros and
|| hardware implementations should return zeros.
||
|| == New MNRET instruction
||
|| This new M-mode only instruction uses the values in `mnepc` and
|| `mnstatus` to return to the program counter and privileged mode of the
|| interrupted context respectively. This instruction also sets the
|| `rnmie` state bit.
||
|| MNRET instruction encoding is same as MRET except with bit 30 set
|| (i.e.,`funct7`=`0111000`).
||
|| == RNMI Operation
||
|| When an RNMI interrupt is detected, the interrupted PC is written to
|| the `mnepc` CSR, the type of RNMI to the `mncause` CSR, and the
|| privilege mode of the interrupted context to the `mnstatus` CSR. An
|| internal microarchitectural state bit `rnmie` is cleared to indicate
|| that processor is in an RNMI handler and cannot take a new RNMI
|| interrupt. The internal `rnmie` bit when clear also disables all
|| other interrupts.
||
|| NOTE: These interrupts are called non-maskable because software cannot
|| mask the interrupts, but for correct operation other instances of the
|| same interrupt must be held off until the handler is completed, hence
|| the internal state bit.
||
|| The core then enters machine-mode and jumps to the RNMI trap handler
|| address.
||
|| The RNMI handler can resume original execution using the new MNRET
|| instruction, which restores the PC from `mnepc`, the privilege mode
|| from `mnstatus`, and also sets the internal `rnmie` state bit, which
|| reenables other interrupts.
||
|| If the hart encounters an exception while the `rnmie` bit is clear, the
|| exception state is written to `mepc` and `mcause`, `mstatus.mpp` is
|| set to M-mode, and the hart jumps to the RNMI exception handler
|| address.
||
|| NOTE: Traps in the RNMI handler can only be resumed if they occur while
|| the handler was servicing an interrupt that occured outside of
|| machine-mode.
||
|| == Interaction with debugger
||
|| The debugger can be configured such that an RNMI event drops the
|| system into the debugger.
||
||
||
||
||
