Enable RDT Configuration

On x86 platforms that support Intel Resource Director Technology (RDT) allocation features such as Cache Allocation Technology (CAT) and Memory Bandwidth Allocation (MBA), the ACRN hypervisor can be used to limit regular VMs that may be over-utilizing common resources such as cache and memory bandwidth relative to their priorities so that the performance of other higher priority VMs (such as RTVMs) is not impacted.

Using RDT includes three steps:

  1. Detect and enumerate RDT allocation capabilities on supported resources such as cache and memory bandwidth.

  2. Set up resource mask array MSRs (model-specific registers) for each CLOS (class of service, which is a resource allocation), basically to limit or allow access to resource usage.

  3. Select the CLOS for the CPU associated with the VM that will apply the resource mask on the CP.

Steps #2 and #3 configure RDT resources for a VM and can be done in two ways:

The following sections discuss how to detect, enumerate capabilities, and configure RDT resources for VMs in the ACRN hypervisor.

For further details, refer to the ACRN RDT high-level design RDT Allocation Feature Supported by Hypervisor and Intel 64 and IA-32 Architectures Software Developer’s Manual, (Section 17.19 Intel Resource Director Technology Allocation Features)

RDT Detection and Resource Capabilities

From the ACRN HV debug shell, use cpuid to detect and identify the resource capabilities. Use the platform’s serial port for the HV shell.

Check if the platform supports RDT with cpuid. First, run cpuid 0x7 0x0; the return value EBX [bit 15] is set to 1 if the platform supports RDT. Next, run cpuid 0x10 0x0 and check the EBX [3-1] bits. EBX [bit 1] indicates that L3 CAT is supported. EBX [bit 2] indicates that L2 CAT is supported. EBX [bit 3] indicates that MBA is supported. To query the capabilities of the supported resources, use the bit position as a subleaf index. For example, run cpuid 0x10 0x2 to query the L2 CAT capability.

ACRN:\>cpuid 0x7 0x0
cpuid leaf: 0x7, subleaf: 0x0, 0x0:0xd39ffffb:0x00000818:0xbc000400

L3/L2 bit encoding:

  • EAX [bit 4:0] reports the length of the cache mask minus one. For example, a value 0xa means the cache mask is 0x7ff.

  • EBX [bit 31:0] reports a bit mask. Each set bit indicates the corresponding unit of the cache allocation that can be used by other entities in the platform (e.g. integrated graphics engine).

  • ECX [bit 2] if set, indicates that cache Code and Data Prioritization Technology is supported.

  • EDX [bit 15:0] reports the maximum CLOS supported for the resource minus one. For example, a value of 0xf means the max CLOS supported is 0x10.

    ACRN:\>cpuid 0x10 0x0
    cpuid leaf: 0x10, subleaf: 0x0, 0x0:0xa:0x0:0x0
    ACRN:\>cpuid 0x10 **0x1**
    cpuid leaf: 0x10, subleaf: 0x1, 0xa:0x600:0x4:0xf

MBA bit encoding:

  • EAX [bit 11:0] reports the maximum MBA throttling value minus one. For example, a value 0x59 means the max delay value is 0x60.

  • EBX [bit 31:0] reserved.

  • ECX [bit 2] reports whether the response of the delay values is linear.

  • EDX [bit 15:0] reports the maximum CLOS supported for the resource minus one. For example, a value of 0x7 means the max CLOS supported is 0x8.

    ACRN:\>cpuid 0x10 0x0
    cpuid leaf: 0x10, subleaf: 0x0, 0x0:0xa:0x0:0x0
    ACRN:\>cpuid 0x10 **0x3**
    cpuid leaf: 0x10, subleaf: 0x3, 0x59:0x0:0x4:0x7


ACRN takes the lowest common CLOS max value between the supported resources as maximum supported CLOS ID. For example, if max CLOS supported by L3 is 16 and MBA is 8, ACRN programs MAX_PLATFORM_CLOS_NUM to 8. ACRN recommends having consistent capabilities across all RDT resources by using a common subset CLOS. This is done in order to minimize misconfiguration errors.

Tuning RDT Resources in HV Debug Shell

This section explains how to configure the RDT resources from the HV debug shell.

  1. Check the pCPU IDs of each VM; the vcpu_list below shows that VM0 is running on pCPU0, and VM1 is running on pCPU1:

    =====    =======    =======    =========    ==========
      0         0          0        PRIMARY       Running
      1         1          0        PRIMARY       Running
  2. Set the resource mask array MSRs for each CLOS with a wrmsr <reg_num> <value>. For example, if you want to restrict VM1 to use the lower 4 ways of LLC cache and you want to allocate the upper 7 ways of LLC to access to VM0, you must first assign a CLOS for each VM (e.g. VM0 is assigned CLOS0 and VM1 CLOS1). Next, resource mask the MSR that corresponds to the CLOS0. In our example, IA32_L3_MASK_BASE + 0 is programmed to 0x7f0. Finally, resource mask the MSR that corresponds to CLOS1. In our example, IA32_L3_MASK_BASE + 1 is set to 0xf.

    ACRN:\>wrmsr  -p1 0xc90  0x7f0
    ACRN:\>wrmsr  -p1 0xc91  0xf
  3. Assign CLOS1 to pCPU1 by programming the MSR IA32_PQR_ASSOC [bit 63:32] (0xc8f) to 0x100000000 to use CLOS1 and assign CLOS0 to pCPU 0 by programming MSR IA32_PQR_ASSOC [bit 63:32] to 0x0. Note that IA32_PQR_ASSOC is per LP MSR and CLOS must be programmed on each LP.

    ACRN:\>wrmsr   -p0   0xc8f    0x000000000 (this is default and can be skipped)
    ACRN:\>wrmsr   -p1   0xc8f    0x100000000

Configure RDT for VM Using VM Configuration

  1. RDT hardware feature is enabled by default on supported platforms. This information can be found using an offline tool that generates a platform-specific XML file that helps ACRN identify RDT-supported platforms. RDT on ACRN is enabled by configuring the FEATURES sub-section of the scenario XML file as in the below example. For details on building ACRN with a scenario, refer to Getting Started Guide.

  2. Once RDT is enabled in the scenario XML file, the next step is to program the desired cache mask or/and the MBA delay value as needed in the scenario file. Each cache mask or MBA delay configuration corresponds to a CLOS ID. For example, if the maximum supported CLOS ID is 4, then 4 cache mask settings needs to be in place where each setting corresponds to a CLOS ID starting from 0. To set the cache masks for 4 CLOS ID and use default delay value for MBA, it can be done as shown in the example below.



    Users can change the mask values, but the cache mask must have continuous bits or a #GP fault can be triggered. Similarly, when programming an MBA delay value, be sure to set the value to less than or equal to the MAX delay value.

  3. Configure each CPU in VMs to a desired CLOS ID in the VM section of the scenario file. Follow RDT detection and resource capabilities to identify the maximum supported CLOS ID that can be used. ACRN uses the lowest common MAX CLOS value among all RDT resources to avoid resource misconfigurations.

    <vm id="0">
       <vm_type readonly="true">PRE_STD_VM</vm_type>
       <name>ACRN PRE-LAUNCHED VM0</name>
       <uuid configurable="0">26c5e0d8-8f8a-47d8-8109-f201ebd61a5e</uuid>


    In ACRN, Lower CLOS always means higher priority (CLOS 0 > CLOS 1 > CLOS 2 > … CLOS n). So, carefully program each VM’s CLOS accordingly.

  4. Careful consideration should be made when assigning vCPU affinity. In a cache isolation configuration, in addition to isolating CAT-capable caches, you must also isolate lower-level caches. In the following example, logical processor #0 and #2 share L1 and L2 caches. In this case, do not assign LP #0 and LP #2 to different VMs that need to do cache isolation. Assign LP #1 and LP #3 with similar consideration:

    # lstopo-no-graphics -v
    Package L#0 (P#0 CPUVendor=GenuineIntel CPUFamilyNumber=6 CPUModelNumber=142)
      L3Cache L#0 (size=3072KB linesize=64 ways=12 Inclusive=1)
        L2Cache L#0 (size=256KB linesize=64 ways=4 Inclusive=0)
          L1dCache L#0 (size=32KB linesize=64 ways=8 Inclusive=0)
            L1iCache L#0 (size=32KB linesize=64 ways=8 Inclusive=0)
              Core L#0 (P#0)
                PU L#0 (P#0)
                PU L#1 (P#2)
        L2Cache L#1 (size=256KB linesize=64 ways=4 Inclusive=0)
          L1dCache L#1 (size=32KB linesize=64 ways=8 Inclusive=0)
            L1iCache L#1 (size=32KB linesize=64 ways=8 Inclusive=0)
              Core L#1 (P#1)
                PU L#2 (P#1)
                PU L#3 (P#3)
  5. Bandwidth control is per-core (not per LP), so max delay values of per-LP CLOS is applied to the core. If HT is turned on, don’t place high priority threads on sibling LPs running lower priority threads.

  6. Based on our scenario, build and install ACRN. See Getting Started Guide for building and installing instructions.

  7. Restart the platform.