Re: [PATCH] KVM: introduce vm's max_halt_poll_ns to debugfs

[Sean Christopherson <seanjc@xxxxxxxxxx>]

On Thu, May 09, 2024, cheng.lin130@xxxxxxxxxx wrote:
> > From: seanjc <seanjc@xxxxxxxxxx>
> > > From: Cheng Lin <cheng.lin130@xxxxxxxxxx>
> > >
> > > Introduce vm's max_halt_poll_ns and override_halt_poll_ns to
> > > debugfs. Provide a way to check and modify them.
> > Why?
> If a vm's max_halt_poll_ns has been set using KVM_CAP_HALT_POLL,
> the module parameter kvm.halt_poll.ns will no longer indicate the maximum
> halt pooling interval for that vm. After introducing these two attributes into
> debugfs, it can be used to check whether the individual configuration of the
> vm is enabled and the working value.

But why is max_halt_poll_ns special enough to warrant debugfs entries?  There is
a _lot_ of state in KVM that is configurable per-VM, it simply isn't feasible to
dump everything into debugfs.

I do think it would be reasonable to capture the max allowed polling time in
the existing tracepoint though, e.g.

diff --git a/include/trace/events/kvm.h b/include/trace/events/kvm.h
index 74e40d5d4af4..7e66e9b2e497 100644
--- a/include/trace/events/kvm.h
+++ b/include/trace/events/kvm.h
@@ -41,24 +41,26 @@ TRACE_EVENT(kvm_userspace_exit,
 );
 
 TRACE_EVENT(kvm_vcpu_wakeup,
-           TP_PROTO(__u64 ns, bool waited, bool valid),
-           TP_ARGS(ns, waited, valid),
+           TP_PROTO(__u64 ns, __u32 max_ns, bool waited, bool valid),
+           TP_ARGS(ns, max_ns, waited, valid),
 
        TP_STRUCT__entry(
                __field(        __u64,          ns              )
+               __field(        __u32,          max_ns          )
                __field(        bool,           waited          )
                __field(        bool,           valid           )
        ),
 
        TP_fast_assign(
                __entry->ns             = ns;
+               __entry->max_ns         = max_ns;
                __entry->waited         = waited;
                __entry->valid          = valid;
        ),
 
-       TP_printk("%s time %lld ns, polling %s",
+       TP_printk("%s time %llu ns (max poll %u ns), polling %s",
                  __entry->waited ? "wait" : "poll",
-                 __entry->ns,
+                 __entry->ns, __entry->max_ns,
                  __entry->valid ? "valid" : "invalid")
 );
 
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 2e388972d856..f093138f3cd7 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3846,7 +3846,8 @@ void kvm_vcpu_halt(struct kvm_vcpu *vcpu)
                }
        }
 
-       trace_kvm_vcpu_wakeup(halt_ns, waited, vcpu_valid_wakeup(vcpu));
+       trace_kvm_vcpu_wakeup(halt_ns, max_halt_poll_ns, waited,
+                             vcpu_valid_wakeup(vcpu));
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_halt);