On 4/26/2024 5:46 AM, Sean Christopherson wrote:
On Thu, Apr 25, 2024, Kan Liang wrote:
On 2024-04-25 4:16 p.m., Mingwei Zhang wrote:
On Thu, Apr 25, 2024 at 9:13 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
It should not happen. For the current implementation, perf rejects all
the !exclude_guest system-wide event creation if a guest with the vPMU
is running.
However, it's possible to create an exclude_guest system-wide event at
any time. KVM cannot use the information from the VM-entry to decide if
there will be active perf events in the VM-exit.
Hmm, why not? If there is any exclude_guest system-wide event,
perf_guest_enter() can return something to tell KVM "hey, some active
host events are swapped out. they are originally in counter #2 and
#3". If so, at the time when perf_guest_enter() returns, KVM will ack
that and keep it in its pmu data structure.
I think it's possible that someone creates !exclude_guest event after
I assume you mean an exclude_guest=1 event? Because perf should be in a state
where it rejects exclude_guest=0 events.
Suppose should be exclude_guest=1 event, the perf event without
exclude_guest attribute would be blocked to create in the v2 patches
which we are working on.
the perf_guest_enter(). The stale information is saved in the KVM. Perf
will schedule the event in the next perf_guest_exit(). KVM will not know it.
Ya, the creation of an event on a CPU that currently has guest PMU state loaded
is what I had in mind when I suggested a callback in my sketch:
: D. Add a perf callback that is invoked from IRQ context when perf wants to
: configure a new PMU-based events, *before* actually programming the MSRs,
: and have KVM's callback put the guest PMU state
when host creates a perf event with exclude_guest attribute which is
used to profile KVM/VMM user space, the vCPU process could work at three
places.
1. in guest state (non-root mode)
2. inside vcpu-loop
3. outside vcpu-loop
Since the PMU state has already been switched to host state, we don't
need to consider the case 3 and only care about cases 1 and 2.
when host creates a perf event with exclude_guest attribute to profile
KVM/VMM user space, an IPI is triggered to enable the perf event
eventually like the following code shows.
event_function_call(event, __perf_event_enable, NULL);
For case 1, a vm-exit is triggered and KVM starts to process the
vm-exit and then run IPI irq handler, exactly speaking
__perf_event_enable() to enable the perf event.
For case 2, the IPI irq handler would preempt the vcpu-loop and call
__perf_event_enable() to enable the perf event.
So IMO KVM just needs to provide a callback to switch guest/host PMU
state, and __perf_event_enable() calls this callback before really
touching PMU MSRs.
It's a similar idea to TIF_NEED_FPU_LOAD, just that instead of a common chunk of
kernel code swapping out the guest state (kernel_fpu_begin()), it's a callback
into KVM.
