On 11/8/2024 5:14 AM, dongli.zhang@xxxxxxxxxx wrote: > Hi Zhao, > > > On 11/6/24 11:52 PM, Zhao Liu wrote: >> (+Dapang & Zide) >> >> Hi Dongli, >> >> On Mon, Nov 04, 2024 at 01:40:17AM -0800, Dongli Zhang wrote: >>> Date: Mon, 4 Nov 2024 01:40:17 -0800 >>> From: Dongli Zhang <dongli.zhang@xxxxxxxxxx> >>> Subject: [PATCH 2/7] target/i386/kvm: introduce 'pmu-cap-disabled' to set >>> KVM_PMU_CAP_DISABLE >>> X-Mailer: git-send-email 2.43.5 >>> >>> The AMD PMU virtualization is not disabled when configuring >>> "-cpu host,-pmu" in the QEMU command line on an AMD server. Neither >>> "-cpu host,-pmu" nor "-cpu EPYC" effectively disables AMD PMU >>> virtualization in such an environment. >>> >>> As a result, VM logs typically show: >>> >>> [ 0.510611] Performance Events: Fam17h+ core perfctr, AMD PMU driver. >>> >>> whereas the expected logs should be: >>> >>> [ 0.596381] Performance Events: PMU not available due to virtualization, using software events only. >>> [ 0.600972] NMI watchdog: Perf NMI watchdog permanently disabled >>> >>> This discrepancy occurs because AMD PMU does not use CPUID to determine >>> whether PMU virtualization is supported. >> >> Intel platform doesn't have this issue since Linux kernel fails to check >> the CPU family & model when "-cpu *,-pmu" option clears PMU version. >> >> The difference between Intel and AMD platforms, however, is that it seems >> Intel hardly ever reaches the “...due virtualization” message, but >> instead reports an error because it recognizes a mismatched family/model. >> >> This may be a drawback of the PMU driver's print message, but the result >> is the same, it prevents the PMU driver from enabling. >> >> So, please mention that KVM_PMU_CAP_DISABLE doesn't change the PMU >> behavior on Intel platform because current "pmu" property works as >> expected. > > Sure. I will mention this in v2. > >> >>> To address this, we introduce a new property, 'pmu-cap-disabled', for KVM >>> acceleration. This property sets KVM_PMU_CAP_DISABLE if >>> KVM_CAP_PMU_CAPABILITY is supported. Note that this feature currently >>> supports only x86 hosts, as KVM_CAP_PMU_CAPABILITY is used exclusively for >>> x86 systems. >>> >>> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx> >>> --- >>> Another previous solution to re-use '-cpu host,-pmu': >>> https://urldefense.com/v3/__https://lore.kernel.org/all/20221119122901.2469-1-dongli.zhang@xxxxxxxxxx/__;!!ACWV5N9M2RV99hQ!Nm8Db-mwBoMIwKkRqzC9kgNi5uZ7SCIf43zUBn92Ar_NEbLXq-ZkrDDvpvDQ4cnS2i4VyKAp6CRVE12bRkMF$ >> >> IMO, I prefer the previous version. This VM-level KVM property is >> difficult to integrate with the existing CPU properties. Pls refer later >> comments for reasons. >> >>> accel/kvm/kvm-all.c | 1 + >>> include/sysemu/kvm_int.h | 1 + >>> qemu-options.hx | 9 ++++++- >>> target/i386/cpu.c | 2 +- >>> target/i386/kvm/kvm.c | 52 ++++++++++++++++++++++++++++++++++++++ >>> target/i386/kvm/kvm_i386.h | 2 ++ >>> 6 files changed, 65 insertions(+), 2 deletions(-) >>> >>> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c >>> index 801cff16a5..8b5ba45cf7 100644 >>> --- a/accel/kvm/kvm-all.c >>> +++ b/accel/kvm/kvm-all.c >>> @@ -3933,6 +3933,7 @@ static void kvm_accel_instance_init(Object *obj) >>> s->xen_evtchn_max_pirq = 256; >>> s->device = NULL; >>> s->msr_energy.enable = false; >>> + s->pmu_cap_disabled = false; >>> } >> >> The CPU property "pmu" also defaults to "false"...but: >> >> * max CPU would override this and try to enable PMU by default in >> max_x86_cpu_initfn(). >> >> * Other named CPU models keep the default setting to avoid affecting >> the migration. >> >> The pmu_cap_disabled and “pmu” property look unbound and unassociated, >> so this can cause the conflict when they are not synchronized. For >> example, >> >> -cpu host -accel kvm,pmu-cap-disabled=on >> >> The above options will fail to launch a VM (on Intel platform). >> >> Ideally, the “pmu” property and pmu-cap-disabled should be bound to each >> other and be consistent. But it's not easy because: >> - There is no proper way to have pmu_cap_disabled set different default >> values (e.g., "false" for max CPU and "true" for named CPU models) >> based on different CPU models. >> - And, no proper place to check the consistency of pmu_cap_disabled and >> enable_pmu. >> >> Therefore, I prefer your previous approach, to reuse current CPU "pmu" >> property. > > Thank you very much for the suggestion and reasons. > > I am going to follow your suggestion to switch back to the previous solution in v2. > >> >> Further, considering that this is currently the only case that needs to >> to set the VM level's capability in the CPU context, there is no need to >> introduce a new kvm interface (in your previous patch), which can instead >> be set in kvm_cpu_realizefn(), like: >> >> >> diff --git a/target/i386/kvm/kvm-cpu.c b/target/i386/kvm/kvm-cpu.c >> index 99d1941cf51c..05e9c9a1a0cf 100644 >> --- a/target/i386/kvm/kvm-cpu.c >> +++ b/target/i386/kvm/kvm-cpu.c >> @@ -42,6 +42,8 @@ static bool kvm_cpu_realizefn(CPUState *cs, Error **errp) >> { >> X86CPU *cpu = X86_CPU(cs); >> CPUX86State *env = &cpu->env; >> + KVMState *s = kvm_state; >> + static bool first = true; >> bool ret; >> >> /* >> @@ -63,6 +65,29 @@ static bool kvm_cpu_realizefn(CPUState *cs, Error **errp) >> * check/update ucode_rev, phys_bits, guest_phys_bits, mwait >> * cpu_common_realizefn() (via xcc->parent_realize) >> */ >> + >> + if (first) { >> + first = false; >> + >> + /* >> + * Since Linux v5.18, KVM provides a VM-level capability to easily >> + * disable PMUs; however, QEMU has been providing PMU property per >> + * CPU since v1.6. In order to accommodate both, have to configure >> + * the VM-level capability here. >> + */ >> + if (!cpu->enable_pmu && >> + kvm_check_extension(s, KVM_CAP_PMU_CAPABILITY)) { >> + int r = kvm_vm_enable_cap(s, KVM_CAP_PMU_CAPABILITY, 0, >> + KVM_PMU_CAP_DISABLE); >> + >> + if (r < 0) { >> + error_setg(errp, "kvm: Failed to disable pmu cap: %s", >> + strerror(-r)); >> + return false; >> + } >> + } >> + } >> + >> if (cpu->max_features) { >> if (enable_cpu_pm) { >> if (kvm_has_waitpkg()) { >> --- > > Sure. I will limit the change within only x86 + KVM. > >> >> In addition, if PMU is disabled, why not mask the perf related bits in >> 8000_0001_ECX? :) >> > > My fault. I have masked only 0x80000022, and I forgot 0x80000001 for AMD. > > Thank you very much for the reminder. > > I think this may not show a functional impact because the guest kernel would anyway not register the core PMU because of being unable to access the PMC MSRs as a result of KVM_PMU_CAP_DISABLE but its the right thing to do because the guest OS may not always be Linux-based :)
