Re: [PATCH 1/1] KVM: x86/vPMU: Check PMU is enabled for vCPU before searching for PMC

Jim Mattson
2023-11-09T20:13:27Z

On Thu, Nov 9, 2023 at 10:24 AM Konstantin Khorenko
<khorenko@virtuozzo.com> wrote:
>
> The following 2 mainstream patches have introduced extra
> events accounting:
>
> 018d70ffcfec ("KVM: x86: Update vPMCs when retiring branch instructions")
> 9cd803d496e7 ("KVM: x86: Update vPMCs when retiring instructions")
>
> kvm_pmu_trigger_event() iterates over all PMCs looking for enabled and
> this appeared to be fast on Intel CPUs and quite expensive for AMD CPUs.
>
> kvm_pmu_trigger_event() can be optimized not to iterate over all PMCs in
> the following cases:
>
> * if PMU is completely disabled for a VM, which is the default
> configuration
> * if PMU v2 is enabled, but no PMCs are configured
>
> For Intel CPUs:
> * By default PMU is disabled for KVM VMs (<pmu state='off'/> or absent
> in the VM config xml which results in "-cpu pmu=off" qemu option).
> In this case pmu->version is reported as 0 for the appropriate vCPU.
>
> * According to Intel® 64 and IA-32 Architectures Software Developer’s
> Manual PMU version 2 and higher provide IA32_PERF_GLOBAL_CTRL MSR
> which in particular contains bits which can be used for efficient
> detection which fixed-function performance and general-purpose
> performance monitoring counters are enabled at the moment.
>
> * Searching for enabled PMCs is fast and the optimization does not
> bring noticeable performance increase.
>
> For AMD CPUs:
> * For CPUs older than Zen 4 pmu->version is always reported as "1" for
> the appropriate vCPU, no matter if PMU is disabled for KVM VMs
> (<pmu state='off'/>) or enabled.
> So for "old" CPUs currently it's impossible to detect when PMU is
> disabled for a VM and skip the iteration by PMCs efficiently.
>
> * Since Zen 4 AMD CPUs support PMU v2 and in this case pmu->version
> should be reported as "2" and IA32_PERF_GLOBAL_CTRL MSR is available
> and can be used for fast and efficient check for enabled PMCs.
> https://www.phoronix.com/news/AMD-PerfMonV2-Linux-Patches
> https://www.phoronix.com/news/AMD-PerfMonV2-Guests-KVM
>
> * Optimized preliminary check for enabled PMCs on AMD Zen 4 CPUs
> should give quite noticeable performance improvement.
>
> AMD performance results:
> CPU: AMD Zen 3 (three!): AMD EPYC 7443P 24-Core Processor
>
> * The test binary is run inside an AlmaLinux 9 VM with their stock kernel
> 5.14.0-284.11.1.el9_2.x86_64.
> * Test binary checks the CPUID instractions rate (instructions per sec).
> * Default VM config (PMU is off, pmu->version is reported as 1).
> * The Host runs the kernel under test.
>
> # for i in 1 2 3 4 5 ; do ./at_cpu_cpuid.pub ; done | \
> awk -e '{print $4;}' | \
> cut -f1 --delimiter='.' | \
> ./avg.sh
>
> Measurements:
> 1. Host runs stock latest mainstream kernel commit 305230142ae0.
> 2. Host runs same mainstream kernel + current patch.
> 3. Host runs same mainstream kernel + current patch + force
> guest_pmu_is_enabled() to always return "false" using following change:
>
> - if (pmu->version >= 2 && !(pmu->global_ctrl & ~pmu->global_ctrl_mask))
> + if (pmu->version == 1 && !(pmu->global_ctrl & ~pmu->global_ctrl_mask))
>
> --------------------------------------
> | Kernels | CPUID rate |
> --------------------------------------
> | 1. | 1360250 |
> | 2. | 1365536 (+ 0.4%) |
> | 3. | 1541850 (+13.4%) |
> --------------------------------------
>
> Measurement (2) gives some fluctuation, the performance is not increased
> because the test was done on a Zen 3 CPU, so we are unable to use fast
> check for active PMCs.
> Measurement (3) shows expected performance boost on a Zen 4 CPU under
> the same test.
>
> Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
> ---
> arch/x86/kvm/pmu.c | 26 ++++++++++++++++++++++++++
> 1 file changed, 26 insertions(+)
>
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 9ae07db6f0f6..290d407f339b 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -731,12 +731,38 @@ static inline bool cpl_is_matched(struct kvm_pmc *pmc)
> return (static_call(kvm_x86_get_cpl)(pmc->vcpu) == 0) ? select_os : select_user;
> }
>
> +static inline bool guest_pmu_is_enabled(struct kvm_pmu *pmu)
> +{
> + /*
> + * Currently VMs do not have PMU settings in configs which defaults
> + * to "pmu=off".
> + *
> + * For Intel currently this means pmu->version will be 0.
> + * For AMD currently PMU cannot be disabled:

Isn't that what KVM_PMU_CAP_DISABLE is for?