On 8/7/2025 1:33 AM, Sean Christopherson wrote: > On Wed, Aug 06, 2025, Dapeng Mi wrote: >> On 8/6/2025 3:05 AM, Sean Christopherson wrote: >>> Acquire SRCU in the VM-Exit fastpath if and only if KVM needs to check the >>> PMU event filter, to further trim the amount of code that is executed with >>> SRCU protection in the fastpath. Counter-intuitively, holding SRCU can do >>> more harm than good due to masking potential bugs, and introducing a new >>> SRCU-protected asset to code reachable via kvm_skip_emulated_instruction() >>> would be quite notable, i.e. definitely worth auditing. >>> >>> E.g. the primary user of kvm->srcu is KVM's memslots, accessing memslots >>> all but guarantees guest memory may be accessed, accessing guest memory >>> can fault, and page faults might sleep, which isn't allowed while IRQs are >>> disabled. Not acquiring SRCU means the (hypothetical) illegal sleep would >>> be flagged when running with PROVE_RCU=y, even if DEBUG_ATOMIC_SLEEP=n. >>> >>> Note, performance is NOT a motivating factor, as SRCU lock/unlock only >>> adds ~15 cycles of latency to fastpath VM-Exits. I.e. overhead isn't a >>> concern _if_ SRCU protection needs to be extended beyond PMU events, e.g. >>> to honor userspace MSR filters. >>> >>> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> >>> --- > ... > >>> @@ -968,12 +968,14 @@ static void kvm_pmu_trigger_event(struct kvm_vcpu *vcpu, >>> (unsigned long *)&pmu->global_ctrl, X86_PMC_IDX_MAX)) >>> return; >>> >>> + idx = srcu_read_lock(&vcpu->kvm->srcu); >> It looks the asset what "kvm->srcu" protects here is >> kvm->arch.pmu_event_filter which is only read by pmc_is_event_allowed(). >> Besides here, pmc_is_event_allowed() is called by reprogram_counter() but >> without srcu_read_lock()/srcu_read_unlock() protection. > No, reprogram_counter() is only called called in the context of KVM_RUN, i.e. with > the vCPU loaded and thus with kvm->srcu already head for read (acquired by > kvm_arch_vcpu_ioctl_run()). Not sure if I understand correctly, but KVM_SET_PMU_EVENT_FILTER ioctl is a VM-level ioctl and it can be set when vCPUs are running. So assume KVM_SET_PMU_EVENT_FILTER ioctl is called at vCPU0 and vCPU1 is running reprogram_counter(). Is it safe without srcu_read_lock()/srcu_read_unlock() protection? > >> So should we shrink the protection range further and move the >> srcu_read_lock()/srcu_read_unlock() pair into pmc_is_event_allowed() >> helper? The side effect is it would bring some extra overhead since >> srcu_read_lock()/srcu_read_unlock() could be called multiple times. > No, I don't think it's worth getting that precise. As you note, there will be > extra overhead, and it could actually become non-trivial amount of overhead, > albeit in a somewhat pathological scenario. And cpl_is_matched() is easy to > audit, i.e. is very low risk with respect to having "bad" behavior that's hidden > by virtue of holding SRCU. > > E.g. if the guest is using all general purpose PMCs to count instructions > retired, then KVM would acquire/release SRCU 8+ times. On Intel, the fastpath > can run in <800 cycles. Adding 8 * 2 full memory barriers (difficult to measure, > but somewhere in the neighborhood of ~10 cycles per barrier) would increase the > latency by 10-20%. > > Again, that's an extreme scenario, but since there's almost nothing to gain from > pushing SRCU acquisition into the filter checks, I don't see any reason to go > with an approach that we *know* is sub-optimal. Yeah, indeed. If there is no need to add srcu_read_lock()/srcu_read_unlock() protection in reprogram_counter(), I'm good with this. Thanks. > >> An alternative could be to add srcu_read_lock()/srcu_read_unlock() around >> pmc_is_event_allowed() in reprogram_counter() helper as well. > As above, there's no need to modify reprogram_counter(). I don't see any future > where reprogram_counter() would be safe to call in the fastpath, there's simply > too much going on, i.e. I think reprogram_counter() will always be a non-issue.
