Re: [PATCH v2 1/6] KVM: x86/pmu: Setup pmc->eventsel for fixed PMCs

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On 7/12/2021 3:50 am, Jim Mattson wrote:
On Mon, Nov 29, 2021 at 11:42 PM Like Xu <like.xu.linux@xxxxxxxxx> wrote:

From: Like Xu <likexu@xxxxxxxxxxx>

The current pmc->eventsel for fixed counter is underutilised. The
pmc->eventsel can be setup for all known available fixed counters
since we have mapping between fixed pmc index and
the intel_arch_events array.

Either gp or fixed counter, it will simplify the later checks for
consistency between eventsel and perf_hw_id.

Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
---
  arch/x86/kvm/vmx/pmu_intel.c | 16 ++++++++++++++++
  1 file changed, 16 insertions(+)

diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
index 1b7456b2177b..b7ab5fd03681 100644
--- a/arch/x86/kvm/vmx/pmu_intel.c
+++ b/arch/x86/kvm/vmx/pmu_intel.c
@@ -459,6 +459,21 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu,
struct msr_data *msr_info)
         return 1;
  }

+static void setup_fixed_pmc_eventsel(struct kvm_pmu *pmu)
+{
+       size_t size = ARRAY_SIZE(fixed_pmc_events);
+       struct kvm_pmc *pmc;
+       u32 event;
+       int i;
+
+       for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
+               pmc = &pmu->fixed_counters[i];
+               event = fixed_pmc_events[array_index_nospec(i, size)];


How do we know that i < size? For example, Ice Lake supports 4
fixed counters, but fixed_pmc_events only has three entries.

With the help of macro MAX_FIXED_COUNTERS,
the fourth or more fixed counter is currently not supported in KVM.

If the user space sets a super set of CPUID supported by KVM,
any pmu emulation failure is to be expected, right ?

Waiting for more comments from you on this patch set.



+               pmc->eventsel = (intel_arch_events[event].unit_mask << 8) |
+                       intel_arch_events[event].eventsel;
+       }
+}
+







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