[PATCH RFC 0/2] VIRQ (Virtual IRQ) layer to support paravirtual interrupt delivery

[Raymond Mao]

From: Raymond Mao <raymond.mao at riscstar.com>

This RFC proposes a lightweight VIRQ courier/routing layer for
OpenSBI to support paravirtual / trap-n-emulate style interrupt
delivery to S-mode payloads, while keeping host physical interrupts
handled in M-mode.

# VIRQ High-level design

At a high-level the VIRQ layer is composed of three major parts:

1. VIRQ mapping and allocation
   - Provide a stable mapping between a host interrupt endpoint
     (chip_uid, hwirq) and a VIRQ number.
   - VIRQ number allocation uses a growable bitmap.

2. HWIRQ->Domain route rules
   - Route rules are described in DeviceTree under the OpenSBI domain
     configuration using:
       'opensbi,host-irqs = <first_hwirq count> ...;'
   - Each <first_hwirq count> entry is converted into an internal
     inclusive range [first .. first+count-1] and cached as a route
     rule.
   - Default behavior: if an asserted HWIRQ does not match any route
     rule, it is routed to the root domain.

3. Per-(domain,hart) pending queue couriering

   - Each domain maintains a per-hart ring buffer queue of pending
     VIRQs.
   - On an asserted HWIRQ, the courier handler:
       maps (chip_uid,hwirq) -> VIRQ;
       finds destination domain via route rules;
       masks the host HWIRQ (to avoid level-trigger storms);
       pushes the VIRQ into the per-(domain,hart) pending queue;
       injects an SSE event to notify the destination S-mode payload
   - In S-mode, the payload’s SSE trap handler:
       issues an ecall to pop pending VIRQ from the per-hart queue;
       runs its ISR for the device;
       issues an ecall to complete the VIRQ, which unmasks the
       corresponding host HWIRQ

# VIRQ ECALL extension

Add a vendor-defined SBI extension ecall for VIRQ.
This allows S-mode payload to pop/complete the next pending VIRQ has
couried into the current domain.

# Reference DT overlay for introducing the route rules:

A reference DT overlay can be used to add route rules to a domain
using 'opensbi,host-irqs' property.

For example, to route UART RX HWIRQ 10 to domain1:
```
/ {
  fragment at 0 {
    target-path = "/chosen";
    __overlay__ {
      opensbi-domains {
        compatible = "opensbi,domain,config";

        domain1: domain1 {
          compatible = "opensbi,domain,instance";
          possible-harts = <0x1>;      /* cpu0 */
          regions = <...>;     /* minimal executable + device MMIO */
          opensbi,host-irqs = <10 1>; /* route hwirq 10 to domain1 */
        };
      };
    };
  };
};
```
A sample DT overlay dts for reference is attached together with the
RFC.

# Boot-time and runtime flow

1. Cold boot flow (M-mode):
   - During FDT domain parsing, extract 'opensbi,host-irqs' from each
     domain instance and cache the route rules in VIRQ layer.
   - Initialize host irqchip drivers and register the VIRQ courier
     handler.
2. When a host HWIRQ is asserted and trapped/handled in M-mode by the
   host irqchip driver.
   - The courier handler maps (chip_uid,hwirq) to a stable VIRQ number.
   - The courier handler routes the interrupt to the destination
     domain using cached route rules (or defaults to root domain if no
     rules match).
   - The courier handler masks the host HWIRQ and pushes the VIRQ into
     the per-hart pending queue of the routed domain.
   - The courier handler injects an SSE event to notify S-mode.
   - The S-mode payload traps the SSE, pops the pending VIRQ(s) via
     ecall, and runs the ISR.
   - The S-mode payload completes the interrupt via ecall, which
     unmasks the host HWIRQ, allowing further interrupts.

Raymond Mao (2):
  lib: sbi: Prototype of Virtual IRQ (VIRQ) layer for
    mapping/routing/courier/ IRQs
  platform: generic: Add sample dts overlay for testing hwirq/domain
    route rules

 include/sbi/sbi_virq.h                      | 401 ++++++++++++++++++++
 platform/generic/virt/hwirq_bind_domain.dts |  79 ++++
 2 files changed, 480 insertions(+)
 create mode 100644 include/sbi/sbi_virq.h
 create mode 100644 platform/generic/virt/hwirq_bind_domain.dts

-- 
2.25.1