On Thu, Nov 07, 2024, Paolo Bonzini wrote: > On Fri, Nov 1, 2024 at 6:15 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > I'm not opposed to letting userspace say "no debugfs for me", but I don't know > > that a module param is the right way to go. It's obviously quite easy to > > implement and maintain (in code), but I'm mildly concerned that it'll have limited > > usefulness and/or lead to bad user experiences, e.g. because people turn off debugfs > > for startup latency without entirely realizing what they're sacrificing. > > What are they sacrificing? :) For all intents and purposes, the ability to get an per-VM and per-vCPU information from an arbitrary shell. > The per-VM statistics information is also accessible without debugfs, even > though kvm_stat does not support it. I assume you're referring to KVM_GET_STATS_FD? That's not easy to get at from the shell. If a host is running a single VM, then the per-VM directories aren't needed. But I would be very, very surprised if there's a legitimate use case for running a single VM, with debugfs, that cares deeply about the boot latency of that one VM. FWIW, I would be wholeheartedly in favor of providing tooling to get at stats via KVM_GET_STATS_FD, e.g. given a VM's PID. But then I think it would make sense to have CONFIG_KVM_DEBUGFS, not a module param. > However I'd make the module parameter read-only, so you don't have > half-and-half setups. And maybe even in this mode we should create the > directory anyway to hold the vcpu%d/pid files, which are not > accessible in other ways. > > > One potentially terrible idea would be to setup debugfs asynchronously, so that > > the VM is runnable asap, but userspace still gets full debugfs information. The > > two big wrinkles would be the vCPU debugfs creation and kvm_uevent_notify_change() > > (or at least the STATS_PATH event) would both need to be asynchronous as well. > > STATS_PATH is easy because you can create the toplevel directory > synchronously; same for vCPUs. I'd be willing to at least see what a > patch looks like. Ah, creating the directories synchrously would definitely simplify things.
