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+[[smcdeleg]]
+== "Smcdeleg/Ssccfg" Counter Delegation Extensions, Version 1.0
+
+In modern “Rich OS” environments, hardware performance monitoring
+resources are managed by the kernel, kernel driver, and/or hypervisor.
+Counters may be configured with differing scopes, in some cases counting
+events system-wide, while in others counting events on behalf of a
+single virtual machine or application. In such environments, the latency
+of counter writes has a direct impact on overall profiling overhead as a
+result of frequent counter writes during:
+
+. Sample collection, to clear overflow indication, and reload overflowed
+counter(s)
+. Context switch, between processes, threads, containers, or virtual
+machines
+
+These extensions provide a means for M-mode to allow writing select
+counters and event selectors from S/HS-mode. The purpose is to avert
+transitions to and from M-mode that add latency to these performance
+critical supervisor/hypervisor code sections. These extensions also
+defines one new CSR, scountinhibit.
+
+For a Machine-level environment, extension *Smcdeleg* (‘Sm’ for
+Privileged architecture and Machine-level extension, ‘cdeleg’ for
+Counter Delegation) encompasses all added CSRs and all behavior
+modifications for a hart, over all privilege levels. For a
+Supervisor-level environment, extension *Ssccfg* (‘Ss’ for Privileged
+architecture and Supervisor-level extension, ‘ccfg’ for Counter
+Configuration) provides access to delegated counters, and to new
+supervisor-level state.  For a RISC-V hardware platform, Smcdeleg and
+Ssccfg must always be implemented in tandem.
+
+=== Counter Delegation
+
+The `mcounteren` register allows M-mode to provide the next-lower
+privilege mode with read access to select counters. When the Smcdeleg/Ssccfg
+extensions are enabled (`menvcfg`.CDE=1), it further allows M-mode to delegate select
+counters to S-mode.
+
+The `siselect` (and `vsiselect`) index range 0x40-0x5F is reserved for
+delegated counter access. When a counter _i_ is delegated
+(`mcounteren`[_i_]=1 and `menvcfg`.CDE=1), the register state associated
+with counter _i_ can be read or written via `sireg*`, while `siselect` holds
+0x40+__i__. The counter state accessible via alias CSRs is shown in
+the table below.
+
+.Indirect HPM State Mappings
+[#indirect-hpm-state-mappings]
+[width="100%",cols="21%,20%,21%,18%,20%",options="header",]
+|===
+|*`siselect` value* |*`sireg*` |*`sireg4`* |*`sireg2`* |*`sireg5`*
+|0x40 |`cycle`^1^ |`cycleh`^1^ |`cyclecfg`^14^ |`cyclecfgh`^14^
+|0x41 4+^|_See below_
+|0x42 |`instret`^1^ |`instreth`^1^ |`instretcfg`^14^ |`instretcfgh`^14^
+|0x43 |`hpmcounter3`^2^ |`hpmcounter3h`^2^ |`hpmevent3`^2^ |`hpmevent3h`^23^
+|… |… |… |… |…
+|0x5F |`hpmcounter31`^2^ |`hpmcounter31h`^2^ |`hpmevent31`^2^ |`hpmevent31h`^23^
+|===
+
+^1^ Depends on Zicntr support +
+^2^ Depends on Zihpm support +
+^3^ Depends on Sscofpmf support +
+^4^ Depends on Smcntrpmf support
+
+`hpmevent__i__` may represent a subset of the state accessed by the `mhpmevent__i__` register. Specifically, if Sscofpmf is implemented, event selector bit
+62 (MINH) is read-only 0 when accessed through `sireg*`.
+
+Likewise, `cyclecfg` and `instretcfg` may represent a subset of the state accessed by the `mcyclecfg` and `minstretcfg` registers, respectively. If
+Smcntrpmf is implemented, counter configuration register bit 62 (MINH) is read-only 0 when accessed through `sireg*`.
+
+If extension Smstateen is implemented, refer to extensions Smcsrind/Sscsrind (<<indirect-csr>>) for how setting bit 60 of CSR
+`mstateen0` to zero prevents access to registers `siselect`, `sireg*`,
+`vsiselect`, and `vsireg*` from privileged modes less privileged than
+M-mode, and likewise how setting bit 60 of `hstateen0` to zero prevents
+access to `siselect` and `sireg*` (really `vsiselect` and `vsireg*`) from
+VS-mode.
+
+The remaining rules of this section apply only when access to a CSR is
+not blocked by `mstateen0`[60] = 0 or `hstateen0`[60] = 0.
+
+While the privilege mode is M or S and `siselect` holds a value in the
+range 0x40-0x5F, illegal-instruction exceptions are raised for the
+following cases:
+
+* attempts to access any `sireg*` when `menvcfg`.CDE = 0;
+* attempts to access `sireg3` or `sireg6`;
+* attempts to access `sireg4` or `sireg5` when XLEN = 64;
+* attempts to access `sireg*` when `siselect` = 0x41, or when the counter
+selected by `siselect` is not delegated to S-mode (the corresponding bit
+in `mcounteren` = 0).
+
+NOTE: _The memory-mapped `mtime` register is not a performance monitoring
+counter to be managed by supervisor software, hence the special
+treatment of `siselect` value 0x41 described above._
+
+For each `siselect` and `sireg*` combination defined in <<indirect-hpm-state-mappings>>, the table
+further indicates the extensions upon which the underlying counter state
+depends. If any extension upon which the underlying state depends is not
+implemented, an attempt from M or S mode to access the given state
+through `sireg*` raises an illegal-instruction exception.
+
+If the hypervisor (H) extension is also implemented, then as specified
+by extensions Smcsrind/Sscsrind, a virtual-instruction  exception is
+raised for attempts from VS-mode or VU-mode to directly access `vsiselect`
+or `vsireg*`, or attempts from VU-mode to access `siselect` or `sireg*`. Furthermore, while `vsiselect` holds a value in the range 0x40-0x5F:
+
+* An attempt to access any `vsireg*` from M or S mode raises an illegal-instruction exception.
+* An attempt from VS-mode to access any `sireg*` (really `vsireg*`) raises an illegal-instruction exception if `menvcfg`.CDE = 0, or a
+virtual-instruction exception if `menvcfg`.CDE = 1.
+
+=== Supervisor Counter Inhibit (`scountinhibit`) Register
+
+Smcdeleg/Ssccfg defines a new `scountinhibit` register, a masked alias of
+`mcountinhibit`. For counters delegated to S-mode, the associated
+`mcountinhibit` bits can be accessed via `scountinhibit`. For counters not
+delegated to S-mode, the associated bits in `scountinhibit` are read-only
+zero.
+
+When `menvcfg`.CDE=0, attempts to access `scountinhibit` raise an illegal-instruction
+exception. When Supervisor Counter Delegation
+is enabled, attempts to access `scountinhibit` from VS-mode or VU-mode
+raise a virtual-instruction  exception.
+
+=== Virtualizing `scountovf`
+
+For implementations that support Smcdeleg/Ssccfg, Sscofpmf, and the H
+extension, when `menvcfg`.CDE=1, attempts to read `scountovf` from VS-mode
+or VU-mode raise a virtual-instruction  exception.
+
+=== Virtualizing Local-Counter-Overflow Interrupts
+
+For implementations that support Smcdeleg, Sscofpmf, and Smaia, the
+local-counter-overflow interrupt (LCOFI) bit (bit 13) in each of CSRs
+`mvip` and `mvien` is implemented and writable.
+
+For implementations that support Smcdeleg/Ssccfg, Sscofpmf,
+Smaia/Ssaia, and the H extension, the LCOFI bit (bit 13) in each of `hvip`
+and `hvien` is implemented and writable.
+
+[NOTE]
+====
+_The `hvip` register is defined by the hypervisor (H) extension, while the `mvien` and `hvien` registers are defined by the Smaia/Ssaia extensions._
+
+_By virtue of implementing `hvip`.LCOFI, it is implicit that the LCOFI bit (bit 13) in each of `vsie` and `vsip` is also implemented._
+
+_Requiring support for the LCOFI bits listed above ensures that virtual LCOFIs can be delivered to an OS running in S-mode, and to a guest OS running in VS-mode. It is optional whether the LCOFI bit (bit 13) in each of `mideleg` and `hideleg`, which allows all LCOFIs to be delegated to S-mode and VS-mode, respectively, is implemented and writable._
+====