My name is Jan and i am currently writing my master’s thesis.
I am using the Kvm Api and try to integrate it as an instruction set simulator in a SystemC environment.
I need some mechanism to count executed instructions in the guest (or cycles).
Currently I am trying to use the emulated PMU cycle counter in the guest to get the number of executed cycles in the guest.
I am working on Arm64 and use Linux Kernel 4.14.33.
I create the PMU device without creating a in-kernel vgic.
Basically I create a vcpu and run some bare metal code.
For convienence, I append the critical assembler snippet.
I configure the counter, then start the counter, execute 3 or 4 dummy instructions and read the counter again and then exit the guest with an exit_mmio.
I assumed the value should be a very small number, as the guest only executed a few instructions.
The thing is as I read the counter, the value is something like 2970 or 0 (changes in each run).
So to me it looks like the counter is also counting the cycles for instruction emulation in the host, am I right?
Is it possible to just count the cycles in the guest from the guests’s point of view?
I read the kvm-api.txt Documentation and the other documents a few times and tried different approaches, so this forum is my last resort.
Thanks in advance!
// we are in el1
// init system registers
LDR X1, =0x30C50838
MSR SCTLR_EL1, X1
// enable access to pmu counters from el0
mov x0, 0xff
mrs x1, currentel
mrs x7, pmuserenr_el0
orr x7, x7, #0b1111
msr pmuserenr_el0, x7
// set pmcr register (control register)
//enable long counter, count every cycle and enable counters
mrs x5, pmcr_el0
orr x5, x5, #0b1
orr x5, x5, #(1<<6)
eor x5, x5, #(1<<3)
eor x5, x5, #(1<<5)
msr pmcr_el0, x5
// read mvccfiltr register (only enable counting of el1)
mrs x6, pmccfiltr_el0
mov x6, #(1<<30)
msr pmccfiltr_el0, x6
// get interrupt configuration and clear overflow bit
mrs x9, pmintenset_el1
mov x8, #(1<<31)
msr pmovsclr_el0, x8
// write counter
mov x0, #0x0
msr pmccntr_el0, x0 // write counter
// enable cycle counter
mov x1, #(1<<31)
msr pmcntenset_el0, x1
mov x0, #0x2 */
// dummy instruction and provoke mmio-exit
mov x1, #0x3
add x2, x0, x1
mov x2, 0x5000
mrs x1, pmccntr_el0
// read overflow
mrs x8, pmovsclr_el0
// provoke mmio exit (0x500 is not mapped)
ldr x3, [x2]