As of September 2020, gem5 needs to be patched in order to use the ARM PMU.
Edit: As of November 2020, gem5 is now patched and it will be included in the next release. Thanks to the developers!
How to patch gem5
This is not a clean patch (very straightforward), and it is more intended to understand how it works. Nonetheless, this is the patch to apply with git apply
from the gem5 source repository:
diff --git i/src/arch/arm/ArmISA.py w/src/arch/arm/ArmISA.py
index 2641ec3fb..3d85c1b75 100644
--- i/src/arch/arm/ArmISA.py
+++ w/src/arch/arm/ArmISA.py
@@ -36,6 +36,7 @@
from m5.params import *
from m5.proxy import *
+from m5.SimObject import SimObject
from m5.objects.ArmPMU import ArmPMU
from m5.objects.ArmSystem import SveVectorLength
from m5.objects.BaseISA import BaseISA
@@ -49,6 +50,8 @@ class ArmISA(BaseISA):
cxx_class = 'ArmISA::ISA'
cxx_header = "arch/arm/isa.hh"
+ generateDeviceTree = SimObject.recurseDeviceTree
+
system = Param.System(Parent.any, "System this ISA object belongs to")
pmu = Param.ArmPMU(NULL, "Performance Monitoring Unit")
diff --git i/src/arch/arm/ArmPMU.py w/src/arch/arm/ArmPMU.py
index 047e908b3..58553fbf9 100644
--- i/src/arch/arm/ArmPMU.py
+++ w/src/arch/arm/ArmPMU.py
@@ -40,6 +40,7 @@ from m5.params import *
from m5.params import isNullPointer
from m5.proxy import *
from m5.objects.Gic import ArmInterruptPin
+from m5.util.fdthelper import *
class ProbeEvent(object):
def __init__(self, pmu, _eventId, obj, *listOfNames):
@@ -76,6 +77,17 @@ class ArmPMU(SimObject):
_events = None
+ def generateDeviceTree(self, state):
+ node = FdtNode("pmu")
+ node.appendCompatible("arm,armv8-pmuv3")
+ # gem5 uses GIC controller interrupt notation, where PPI interrupts
+ # start to 16. However, the Linux kernel start from 0, and used a tag
+ # (set to 1) to indicate the PPI interrupt type.
+ node.append(FdtPropertyWords("interrupts", [
+ 1, int(self.interrupt.num) - 16, 0xf04
+ ]))
+ yield node
+
def addEvent(self, newObject):
if not (isinstance(newObject, ProbeEvent)
or isinstance(newObject, SoftwareIncrement)):
diff --git i/src/cpu/BaseCPU.py w/src/cpu/BaseCPU.py
index ab70d1d7f..66a49a038 100644
--- i/src/cpu/BaseCPU.py
+++ w/src/cpu/BaseCPU.py
@@ -302,6 +302,11 @@ class BaseCPU(ClockedObject):
node.appendPhandle(phandle_key)
cpus_node.append(node)
+ # Generate nodes from the BaseCPU children (and don't add them as
+ # subnode). Please note: this is mainly needed for the ISA class.
+ for child_node in self.recurseDeviceTree(state):
+ yield child_node
+
yield cpus_node
def __init__(self, **kwargs):
What the patch resolves
The Linux kernel uses a Device Tree Blob (DTB), which is a regular file, to declare the hardware on which the kernel is running. This is used to make the kernel portable between different architecture without a recompilation for each hardware change. The DTB follows the Device Tree Reference, and is compiled from a Device Tree Source (DTS) file, a regular text file. You can learn more here and here.
The problem was that the PMU is supposed to be declared to the Linux kernel via the DTB. You can learn more here and here. In a simulated system, because the system is specified by the user, gem5 has to generate a DTB itself to pass to the kernel, so the latter can recognize the simulated hardware. However, the problem is that gem5 does not generate the DTB entry for our PMU.
What the patch does
The patch adds an entry to the ISA and the CPU files to enable DTB generation recursion up to find the PMU. The hierarchy is the following: CPU => ISA => PMU. Then, it adds the generation function in the PMU to generate a unique DTB entry to declare the PMU, with the proper notation for the interrupt declaration in the kernel.
After running a simulation with our patch, we could see the DTS from the DTB like this:
cd m5out
# Decompile the DTB to get the DTS.
dtc -I dtb -O dts system.dtb > system.dts
# Find the PMU entry.
head system.dts
dtc
is the Device Tree Compiler, installed with sudo apt-get install device-tree-compiler
. We end up with this pmu
DTB entry, under the root node (/
):
/dts-v1/;
/ {
#address-cells = <0x02>;
#size-cells = <0x02>;
interrupt-parent = <0x05>;
compatible = "arm,vexpress";
model = "V2P-CA15";
arm,hbi = <0x00>;
arm,vexpress,site = <0x0f>;
memory@80000000 {
device_type = "memory";
reg = <0x00 0x80000000 0x01 0x00>;
};
pmu {
compatible = "arm,armv8-pmuv3";
interrupts = <0x01 0x04 0xf04>;
};
cpus {
#address-cells = <0x01>;
#size-cells = <0x00>;
cpu@0 {
device_type = "cpu";
compatible = "gem5,arm-cpu";
[...]
In the line interrupts = <0x01 0x04 0xf04>;
, 0x01
is used to indicate that the number 0x04
is the number of a PPI interrupt (the one declared with number 20
in gem5, the difference of 16
is explained inside the patch code). The 0xf04
corresponds to a flag (0x4
) indicating that it is a "active high level-sensitive" interrupt and a bit mask (0xf
) indicating that the interrupts should be wired to all PE attached to the GIC. You can learn more here.
If the patch works and your ArmPMU
is declared properly, you should see this message at boot time:
[ 0.239967] hw perfevents: enabled with armv8_pmuv3 PMU driver, 32 counters available