ext-sse: add local unknown nmi event id

[cuiyunhui]

Take ID value 0xffff8001 from the "Global events reserved for future use" range as the SBI_SSE_EVENT_GLOBAL_UNKNOWN_NMI event ID.

[cuiyunhui]

1. When the Kernel system becomes unresponsive, we need to initiate an unknown NMI via BIOS to make the Kernel panic() and thereby generate a vmcore.

2. Need to add an SSE event ID, and add SBI_SSE_EVENT_LOCAL_UNKNOWN_NMI to the existing supported_events[]:
   static const uint32_t supported_events[] = {
   SBI_SSE_EVENT_LOCAL_RAS,
   SBI_SSE_EVENT_LOCAL_DOUBLE_TRAP,
   SBI_SSE_EVENT_GLOBAL_RAS,
   SBI_SSE_EVENT_LOCAL_PMU,
   SBI_SSE_EVENT_LOCAL_SOFTWARE,
   SBI_SSE_EVENT_GLOBAL_SOFTWARE,
   };

3. The SSE eventid is a logical software ID, unrelated to hardware properties, so it is better to add it in the SBI spec rather than ACPI.

4. We will add a standard ID, which is UNKNOWN_NMI, and take the ID value 0xffff8001 from the "Global events reserved for future use" range as the SBI_SSE_EVENT_LOCAL_UNKNOWN_NMI event ID.

5. Kernel RFC patch: https://lore.kernel.org/all/CAAhSdy2e90AhkWaaCWsyEjRpovQbS9eBNpC60bLsLTLqUEWSkw@mail.gmail.com/

@atishp04 @avpatel Could you help review this PR? Thanks.

[atishp04]

lgtm

[atishp04]

How does the BIOS/firmware gets invoked to inject the unknown NMI SSE event ?

[weidongwd]

How does the BIOS/firmware gets invoked to inject the unknown NMI SSE event ?

Generally, an external interrupt is used as an Unknown NMI pin, and an Unknown NMI event is sent to the SBI firmware by triggering this pin. Then the SBI firmware will send SBI_SSE_EVENT_GLOBAL_UNKNOWN_NMI to the kernel.

[cuiyunhui]

As discussed during yesterday's [RISC-V][tech-prs] meeting, the global event has been adjusted to a local event based on feedback, and the corresponding patch has been updated accordingly.

@atishp04 @avpatel Could you help review this PR again? Thanks.

[avpatel]

@atishp04 @cuiyunhui Looks good to me.