Re: [PATCH 3/4] KVM: VMX/pmu: Enable inactive vLBR event in guest LBR MSR emulation

[Sean Christopherson <seanjc@google.com>]

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.