Firstly, thanks for your comments that spewed out around vpmu.
On 23/9/2022 8:13 am, Sean Christopherson wrote:
Force vCPUs to reprogram all counters on a PMU filter change to provide
a sane ABI for userspace. Use the existing KVM_REQ_PMU to do the
programming, and take advantage of the fact that the reprogram_pmi bitmap
fits in a u64 to set all bits in a single atomic update. Note, setting
the bitmap and making the request needs to be done _after_ the SRCU
synchronization to ensure that vCPUs will reprogram using the new filter.
KVM's current "lazy" approach is confusing and non-deterministic. It's
The resolute lazy approach was introduced in patch 03, right after this change.
confusing because, from a developer perspective, the code is buggy as it
makes zero sense to let userspace modify the filter but then not actually
enforce the new filter. The lazy approach is non-deterministic because
KVM enforces the filter whenever a counter is reprogrammed, not just on
guest WRMSRs, i.e. a guest might gain/lose access to an event at random
times depending on what is going on in the host.
Note, the resulting behavior is still non-determinstic while the filter
is in flux. If userspace wants to guarantee deterministic behavior, all
vCPUs should be paused during the filter update.
Fixes: 66bb8a065f5a ("KVM: x86: PMU Event Filter")
Cc: Aaron Lewis <aaronlewis@xxxxxxxxxx>
Jim Mattson <jmattson@xxxxxxxxxx>
miss "Cc:" ?
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
arch/x86/include/asm/kvm_host.h | 11 ++++++++++-
arch/x86/kvm/pmu.c | 15 +++++++++++++--
2 files changed, 23 insertions(+), 3 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index b3ce723efb43..462f041ede9f 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -519,7 +519,16 @@ struct kvm_pmu {
struct kvm_pmc gp_counters[INTEL_PMC_MAX_GENERIC];
struct kvm_pmc fixed_counters[KVM_PMC_MAX_FIXED];
struct irq_work irq_work;
- DECLARE_BITMAP(reprogram_pmi, X86_PMC_IDX_MAX);
+
+ /*
+ * Overlay the bitmap with a 64-bit atomic so that all bits can be
+ * set in a single access, e.g. to reprogram all counters when the PMU
+ * filter changes.
+ */
+ union {
+ DECLARE_BITMAP(reprogram_pmi, X86_PMC_IDX_MAX);
+ atomic64_t __reprogram_pmi;
+ };
DECLARE_BITMAP(all_valid_pmc_idx, X86_PMC_IDX_MAX);
DECLARE_BITMAP(pmc_in_use, X86_PMC_IDX_MAX);
diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index d9b9a0f0db17..4504987cbbe2 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -577,6 +577,8 @@ EXPORT_SYMBOL_GPL(kvm_pmu_trigger_event);
int kvm_vm_ioctl_set_pmu_event_filter(struct kvm *kvm, void __user *argp)
{
struct kvm_pmu_event_filter tmp, *filter;
+ struct kvm_vcpu *vcpu;
+ unsigned long i;
size_t size;
int r;
@@ -613,9 +615,18 @@ int kvm_vm_ioctl_set_pmu_event_filter(struct kvm *kvm, void __user *argp)
mutex_lock(&kvm->lock);
filter = rcu_replace_pointer(kvm->arch.pmu_event_filter, filter,
mutex_is_locked(&kvm->lock));
- mutex_unlock(&kvm->lock);
-
synchronize_srcu_expedited(&kvm->srcu);
The relative order of these two operations has been reversed
mutex_unlock() and synchronize_srcu_expedited()
, extending the execution window of the critical area of "kvm->lock)".
The motivation is also not explicitly stated in the commit message.
+
+ BUILD_BUG_ON(sizeof(((struct kvm_pmu *)0)->reprogram_pmi) >
+ sizeof(((struct kvm_pmu *)0)->__reprogram_pmi));
+
+ kvm_for_each_vcpu(i, vcpu, kvm)
+ atomic64_set(&vcpu_to_pmu(vcpu)->__reprogram_pmi, -1ull);
How about:
bitmap_copy(pmu->reprogram_pmi, pmu->all_valid_pmc_idx, X86_PMC_IDX_MAX);
to avoid further cycles on calls of
"static_call(kvm_x86_pmu_pmc_idx_to_pmc)(pmu, bit)" ?
+
+ kvm_make_all_cpus_request(kvm, KVM_REQ_PMU);
+
+ mutex_unlock(&kvm->lock);
+
r = 0;
cleanup:
kfree(filter);
