sqosid.adoc

Quality-of-Service (QoS) Identifiers

The srmcfg register is an SXLEN-bit read/write register used to configure a Resource Control ID (RCID) and a Monitoring Counter ID (MCID). Both RCID and MCID are WARL fields. The register is formatted as shown in Supervisor Resource Management Configuration (srmcfg) register for SXLEN=64 when SXLEN=64 and Supervisor Resource Management Configuration (srmcfg) register for SXLEN=32 when SXLEN=32. The CSR number is 0x181.

The RCID and MCID accompany each request made by the hart to shared resource controllers. The RCID is used to determine the resource allocations (e.g., cache occupancy limits, memory bandwidth limits, etc.) to enforce. The MCID is used to identify a counter to monitor resource usage.

Supervisor Resource Management Configuration (srmcfg) register for SXLEN=64

{reg: [
  {bits: 12, name: 'RCID'},
  {bits:  4, name: 'WPRI'},
  {bits: 12, name: 'MCID'},
  {bits: 36, name: 'WPRI'},
], config:{lanes: 2, hspace:1024}}

Supervisor Resource Management Configuration (srmcfg) register for SXLEN=32

{reg: [
  {bits: 12, name: 'RCID'},
  {bits:  4, name: 'WPRI'},
  {bits: 12, name: 'MCID'},
  {bits:  4, name: 'WPRI'},
], config:{lanes: 1, hspace:1024}}

The RCID and MCID configured in the srmcfg CSR apply to all privilege modes cite:[PRIV] of software execution on that hart by default, but this behavior may be overridden by future extensions.

If extension Smstateen is implemented together with Ssqosid, then Ssqosid also requires the bit 55 in mstateen0 introduced by Priv 1.14 to be implemented. If bit 55 of mstateen0 is 0, attempts to access srmcfg in privilege modes less privileged than M-mode raise an illegal-instruction exception. If bit 55 of mstateen0 is 1 or if extension Smstateen is not implemented, attempts to access srmcfg when V=1 raise a virtual-instruction exception.

Note

A reset value of 0 is suggested for the RCID field matching resource controllers' default behavior of associating all capacity with RCID=0. The MCID reset value does not affect functionality and may be implementation-defined. Typically, fewer bits are allocated for RCID (e.g., to support tens of RCIDs) than for MCID (e.g., to support hundreds of MCIDs). A common RCID is usually used to group apps or VMs, pooling resource allocations to meet collective SLAs. If an SLA breach occurs, unique MCIDs enable granular monitoring, aiding decisions on resource adjustment, associating a different RCID with a subset of members, or migrating members to other machines. The larger pool of MCIDs speeds up this analysis. The RCID and MCID in srmcfg apply across all privilege levels on the hart. Typically, higher-privilege modes don’t modify srmcfg, as they often serve lower-privileged tasks. If differentiation is needed, higher privilege code can update srmcfg and restore it before returning to a lower privilege level.

Note

In VM environments, hypervisors usually manage resource allocations, keeping the Guest OS out of QoS flows. If needed, the hypervisor can virtualize srmcfg CSR for a VM using the virtual-instruction exceptions triggered upon Guest access. If the direct selection of RCID and MCID by the VM becomes common and emulation overhead is an issue, future extensions may allow VS-mode to use a selector for a hypervisor-configured set of CSRs holding RCID and MCID values designated for that Guest OS use. During context switches, the supervisor may choose to execute with the srmcfg of the outgoing context to attribute the execution to it. Prior to restoring the new context, it switches to the new VM’s srmcfg. The supervisor can also use a separate configuration for execution not to be attributed to either contexts.