On Tue, Jun 27, 2023, Xiong Y Zhang wrote: > > On Sun, Jun 25, 2023, Xiong Y Zhang wrote: > > > > > On Fri, Jun 16, 2023, Xiong Zhang wrote: > > > > /* > > > > * Attempt to re-enable the vLBR event if it was disabled due to > > > > * contention with host LBR usage, i.e. was put into an error state. > > > > * Perf doesn't notify KVM if the host stops using LBRs, i.e. KVM needs > > > > * to manually re-enable the event. > > > > */ > > > > > > > > Which begs the question, why can't there be a notification of some > > > > form that the LBRs are once again available? > > > This is perf scheduler rule. If pinned event couldn't get resource as > > > resource limitation, perf will put it into error state and exclude it > > > from perf scheduler, even if resource available later, perf won't > > > schedule it again as it is in error state, the only way to reschedule > > > it is to enable it again. If non-pinned event couldn't get resource > > > as resource limitation, perf will put it into inactive state, perf > > > will reschedule it automatically once resource is available. vLBR event is per > > process pinned event. > > > > That doesn't answer my question. I get that all of this is subject to perf > > scheduling, I'm asking why perf doesn't communicate directly with KVM to > > coordinate access to LBRs instead of pulling the rug out from under KVM. > Perf doesn't need such notification interface currently, as non-pinned event > will be active automatically once resource available, only pinned event is > still in inactive even if resource available, perf may refuse to add such > interface for KVM usage only. Or maybe perf will be overjoyed that someone is finally proposing a coherent interface. Until we actually try/ask, we'll never know. > > Your other response[1] mostly answered that question, but I want explicit > > documentation on the contract between perf and KVM with respect to LBRs. In > > short, please work with Weijiang to fulfill my request/demand[*] that someone > > document KVM's LBR support, and justify the "design". I am simply not willing to > > take KVM LBR patches until that documentation is provided. > Sure, I will work with Weijiang to supply such documentation. Will this > document be put in Documentation/virt/kvm/x86/ ? Ya, Documentation/virt/kvm/x86/pmu.rst please.
