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Smsdia: Supervisor Domain Interrupt Assignment

Interrupts that occur during the execution of a supervisor domain typically cause a trap to the RDSM in M-mode by default. The RDSM may redirect traps back to a supervisor domain using the MRET instruction. Additionally, the RDSM may assert a virtual interrupt to a supervisor domain. Such virtual interrupts might be asserted as part of handling a real interrupt, such as a RAS interrupt, or they may have no direct connection to any actual interrupt event.

To increase performance, the RDSM can delegate the handling of certain interrupts, like the timer interrupt or the local performance counter overflow interrupt (LCOFI), to the supervisor domain by setting the appropriate mideleg bits. This delegation could necessitate the RDSM to context switch additional state related to such delegated interrupts, including stimecmp and the HPM CSRs.

Some supervisor domains may have devices assigned to them for I/O operations. Devices designed to carry out I/O operations typically signal the completion of an I/O task or an associated error via an external interrupt. These external interrupts might be aggregated and indicated by an interrupt controller linked to the RDSM. The RDSM, in the course of handling an external interrupt, should forward the interrupt to the relevant supervisor domain. The efficiency of handling external interrupts by a supervisor domain can be enhanced if the external interrupts could be directly assigned to the supervisor domain Supervisor Domain Interrupt Assignment. Such direct delivery of external interrupts necessitates a dedicated interrupt controller that can be linked to such supervisor domains. To accommodate such supervisor domains, the system might feature multiple interrupt controllers, enabling an individual interrupt controller to be linked with each supervisor domain. The interrupt controllers could be either an APLIC or an IMSIC, as delineated by the RISC-V Advanced Interrupt Architecture (AIA) cite:[AIA]. In certain systems, both an APLIC and an IMSIC could be linked with a supervisor domain, with the APLIC configured to route interrupts to the associated IMSIC as MSI. Interrupt controllers other than APLIC and/or IMSIC could also serve a supervisor domain.

Supervisor Domain Interrupt Assignment
  +----------------+ +----------------+   +----------------+
  |                | |                |   |                |
  |      SD−0      | |      SD−1      |...|      SD−N      |
  |                | |                |   |                |
  +----------------+ +----------------+   +----------------+
           ^                 ^                    ^
           | SEI-0           | SEI-1              | SEI-N
           |                 |                    |
    +------------+     +------------+       +------------+
    |    SD-0    |     |    SD-1    |       |    SD-N    |
    | APLIC/IMSIC|     | APLIC/IMSIC|       | APLIC/IMSIC|
    +------------+     +------------+       +------------+

Although a variety of interrupt controllers can be activated for external interrupts at the S- and VS-levels, a singular interrupt controller manages external interrupts at the M-level and is perpetually active. The Smsdia extension does not impact external interrupts at the M-level.

When an interrupt controller has an S-level external interrupt awaiting resolution at the S-level on a hart, an S-level-external-interrupt-pending signal is asserted. Certain interrupt controllers, like the IMSIC, support delivering external interrupts directly to the VS-level. An IMSIC, for instance, supports up to 63 guest interrupt register files, each potentially linked to a virtual machine that the hypervisor might schedule for execution on a hart. Each guest interrupt register file can assert a VS-level-external-interrupt-pending signal when a VS-level external interrupt is pending resolution by a guest operating at VS-level. With the introduction of supervisor domains, a multitude of S-level and VS-level external interrupt pending signals might be asserted by the various interrupt controllers connected to a hart.

Interrupt controllers are configured through a memory-mapped register interface, a collection of CSRs, or a blend of both. For instance, software interfaces with an APLIC via a memory-mapped register interface. In another case, software interacts with an IMSIC through a set of S and VS-level CSRs, along with a series of memory-mapped registers.

The Smsdia supports the connection of multiple interrupt controllers to a hart, allowing a supervisor domain to be paired with one of these interrupt controllers. The Smsdia extension uses the msdcfg CSR to signify the active interrupt controller for a supervisor domain. When CSRs are employed to interface with the interrupt controller state, they interact with the state corresponding to the active interrupt controller.

The supervisor domain interrupt controllers connected to a hart may not be identical. For example, each supervisor domain IMSIC may support a different number of external interrupt identities for S- and/or VS-level, and the number of guest interrupt files supported by each supervisor domain IMSIC may not be identical.

Upon scheduling a supervisor domain for execution on a hart, the RDSM requires a mechanism to appoint the linked interrupt controller as the active one for the hart. Selecting an interrupt controller also chooses the linked S and VS-level interrupt pending signals as those detected by the hart for instigating the associated traps. When CSRs are utilized to interface with an interrupt controller’s state, the interaction is with the active interrupt controller.

The RDSM needs a method to be alerted if an external interrupt, whether at the S- or VS-level, is pending for any supervisor domains not currently active on a hart. The RDSM could leverage this notification to inform its scheduling decisions. To facilitate this functionality, the Smsdia extension introduces the msdeip and msdeie CSRs, along with an local supervisor domain external interrupt (LSDEI).

Machine supervisor domain configuration (msdcfg)

The msdcfg is a 32-bit read/write register, formatted as shown in [MSDCFG].

The SDICN field selects an interrupt controller among the plurality of supervisor domain interrupt controllers associated with a hart as the active supervisor domain interrupt controller for S- and VS-level external interrupts when SDICN is the number of an implemented interrupt controller, not zero.

The SDICN field is a WLRL field that must be able to hold a value between 0 and the maximum implemented supervisor domain interrupt controller number, inclusive. If there are no supervisor domain interrupt controllers connected to the hart, then SDICN may be read-only zero.

When SDICN is not the number of an implemented interrupt controller, or is zero then the following rules apply to all privilege modes:

  • The S-level external interrupt pending signal indicated in mip.SEIP is 0.

  • All non-custom values of siselect that access IMSIC registers designate an inaccessible register. When siselect holds the number of an inaccessible register, attempts from M-mode or HS-mode to access sireg raise an illegal instruction exception.

  • Access to CSR stopei raises an illegal instruction exception.

  • The hstatus.vgein field is read-only zero.

  • The VS-level external interrupt pending signals indicated in hgeip are 0.

When the interrupt controller selected by msdcfg.SDICN is an implemented interrupt controller, and is not zero, and is an IMSIC, the following rules apply:

  • The S-level external interrupt pending signal of the selected IMSIC is indicated in mip.SEIP.

  • The siselect and stopei CSRs operate on the registers of the S-level interrupt register file in the selected IMSIC when siselect holds a value that selects an IMSIC register.

  • The VS-level external interrupt pending signals of the selected IMSIC are indicated in the hgeip CSR.

  • The hstatus.VGEIN selects a guest interrupt file in the selected IMSIC and vsiselect and vstopei CSRs operate on the registers of the corresponding guest interrupt file in the selected IMSIC.

When the interrupt controller selected by msdcfg.SDICN is an implemented interrupt controller, is not zero, and is an APLIC, the following rules apply:

  • The S-level external interrupt pending signal of the selected APLIC is indicated in mip.SEIP.

Note

The Smsdia extension provides for directly associating an interrupt controller with up to 63 supervisor domains. The RDSM may emulate interrupt controllers for additional supervisor domains and SDICN is set to 0 for such supervisor domains. To emulate an IMSIC, the RDSM may use the illegal instruction trap on access to the associated CSRs for emulation purposes.

If Smsdia extension is implemented, then msdcfg.SDICN is non-zero at reset and holds the number of an implemented supervisor domain interrupt controller.

Machine supervisor domain external interrupt pending (msdeip/msdeiph)

The msdeip is a MXLEN-bit read-only register, formatted for MXLEN=64 as shown in msdeip register for RV64. When MXLEN=32, msdeiph is a 32-bit read-only register which aliases bits 63:32 of msdeip. When MXLEN=64, msdeiph does not exist.

msdeip register for RV64
{reg: [
  {bits:  1, name: '0'},
  {bits: 63, name: 'Interrupts'},
], config:{lanes: 1, hspace:1024}}

Each bit i in the register summarizes the external interrupts pending in the supervisor domain interrupt controller numbered i.

When the interrupt controller identified by i is an APLIC, the bit i indicates the state of the S-level external interrupt pending signal provided by that APLIC.

When the interrupt controller identified by i is an IMSIC, the bit i indicates the logical OR of the S-level and all VS-level external interrupt pending signals provided by that IMSIC.

The state of the supervisor domain interrupts is visible in the msdeip register even when msdcfg.SDICN is zero or is not the valid number of an implemented interrupt controller.

Machine supervisor domain external interrupt enable (msdeie/msdeieh)

The msdeie is a MXLEN-bit read-write register, formatted for MXLEN=64 as shown in msdeie register for RV64. When MXLEN=32, msdeieh is a 32-bit read-write register which aliases bits 63:32 of msdeie. When MXLEN=64, msdeieh does not exist.

msdeie register for RV64
{reg: [
  {bits:  1, name: '0'},
  {bits: 63, name: 'Interrupts'},
], config:{lanes: 1, hspace:1024}}

The msdeie CSR selects the subset of supervisor domain external interrupts that cause a local supervisor domain external interrupt. The enable bits in msdeie do not affect the S- and VS-level external interrupt pending signals from the interrupt controller selected by msdcfg.SDICN.

Machine and Supervisor Interrupt registers (mip/mie and sip/sie)

The Smsdia extension introduces the local supervisor domain external interrupt (LSDEI). This interrupt is treated as a standard local interrupt that is assigned to bit 16 in the mip, mie, sip, and sie registers. The bit 16 in mip and sip is called LSDEIP and the same bit in mie and sie is called LSDEIE. The mideleg register controls the delegation of LSDEI to S-mode. This interrupt cannot be delegated to VS-mode and bit 16 of hideleg is read-only zero.

The mip.LSDEIP bit is read-only, and is 1 if and only if the bitwise logical AND of CSRs msdeip and msdeie is nonzero in any bit. The sip.LSDEIP bit is read-only, and is 0 if LSDEI is not delegated to S-mode otherwise it returns the value of the mip.LSDEIP when read.

Multiple simultaneous interrupts destined for different privilege modes are handled in decreasing order of destined privilege mode. Multiple simultaneous interrupts destined for the same privilege mode are handled in the following decreasing default priority order: high-priority RAS event, MEI, MSI, MTI, SEI, SSI, STI, LSDEI, SGEI, VSEI, VSSI, VSTI, LCOFI, low-priority RAS event.

Note

The RDSM may use the supervisor domain external interrupt to determine if a supervisor domain has become ready to run since it was last descheduled. When a supervisor domain that has a supervisor domain interrupt controller directly assigned to it, the RDSM updates the msdcfg.SDICN to select that interrupt controller and may clear the bit corresponding to that interrupt controller in msdeie prior to resuming execution of the supervisor domain.

The RDSM may delegate LSDEI to a supervisor domain that may be entrusted by the RDSM to get notified about supervisor domain external interrupts pending for one or more other supervisor domains. Typically, this use case involves a single supervisor domain that is trusted by the RDSM to receive such notifications. The delegation supports optimizing the exit sequence from such supervisor domain by enabling such supervisor domains to voluntarily yield execution in response to pending interrupts for the other supervisor domains.