Hi Jan, On 23/11/2018 12:29, James Morse wrote: > On 23/11/2018 09:36, Jan Bolke wrote: >> 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. > >> 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. > > (some of which are system register writes, which can take a long time) > >> The thing is as I read the counter, the value is something like 2970 or 0 >> (changes in each run). > You are missing some barriers in your assembly snippet. 0 is a good indication > that the code you wanted to measure escaped the measurement-window! This would be true on bare-metal, but for KVM I'm talking rubbish. Turns out KVM traps all these registers, and emulates the lot. kvm_arm_setup_debug() sets MDCR_EL2.TPM so almost all these accesses trap, effectively putting an ISB after each one. >> So to me it looks like the counter is also counting the cycles for instruction >> emulation in the host, am I right? > > I'd assume not, but I don't know anything about the PMU. > Andrew Murray posted a series[0] that did some stuff with starting/stopping the > the counters around the guest, but I think that was just for the host making > measurements of itself, or the guest. > It might be easier to count instructions from outside the guest using perf. I > think Andrew's series is making that more reliable. >> Is it possible to just count the cycles in the guest from the guests’s point of >> view? This is what KVM is doing, by emulating the PMU with perf. kvm_pmu_get_counter_value() looks like it should be doing the right thing... As a sanity check, does the cycle counter work on your test machine?: | perf stat -e cycles -- ls Thanks, James _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
