Re: Resumable NMI proposal

Beeman Strong

Oops, I guess the web interface doesn't include the text from the message I was responding to.  Here it is, from Krste:

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.


|| 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.

On Wed, Jun 1, 2022 at 6:15 PM Beeman Strong via <> wrote:
Reviving this old thread.  What is the status of this proposal?  I'd like to explore using RNMIs for performance counter overflow interrupts, in order to support profiling while interrupts are masked.

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