KVM: x86/pmu: Start stubbing in mediated PMU support

Introduce enable_mediated_pmu as a global variable, with the intent of
exposing it to userspace a vendor module parameter, to control and reflect
mediated vPMU support.  Wire up the perf plumbing to create+release a
mediated PMU, but defer exposing the parameter to userspace until KVM
support for a mediated PMUs is fully landed.

To (a) minimize compatibility issues, (b) to give userspace a chance to
opt out of the restrictive side-effects of perf_create_mediated_pmu(),
and (c) to avoid adding new dependencies between enabling an in-kernel
irqchip and a mediated vPMU, defer "creating" a mediated PMU in perf
until the first vCPU is created.

Regarding userspace compatibility, an alternative solution would be to
make the mediated PMU fully opt-in, e.g. to avoid unexpected failure due
to perf_create_mediated_pmu() failing.  Ironically, that approach creates
an even bigger compatibility issue, as turning on enable_mediated_pmu
would silently break VMMs that don't utilize KVM_CAP_PMU_CAPABILITY (well,
silently until the guest tried to access PMU assets).

Regarding an in-kernel irqchip, create a mediated PMU if and only if the
VM has an in-kernel local APIC, as the mediated PMU will take a hard
dependency on forwarding PMIs to the guest without bouncing through host
userspace.  Silently "drop" the PMU instead of rejecting KVM_CREATE_VCPU,
as KVM's existing vPMU support doesn't function correctly if the local
APIC is emulated by userspace, e.g. PMIs will never be delivered.  I.e.
it's far, far more likely that rejecting KVM_CREATE_VCPU would cause
problems, e.g. for tests or userspace daemons that just want to probe
basic KVM functionality.

Note!  Deliberately make mediated PMU creation "sticky", i.e. don't unwind
it on failure to create a vCPU.  Practically speaking, there's no harm to
having a VM with a mediated PMU and no vCPUs.  To avoid an "impossible" VM
setup, reject KVM_CAP_PMU_CAPABILITY if a mediated PMU has been created,
i.e. don't let userspace disable PMU support after failed vCPU creation
(with PMU support enabled).

Defer vendor specific requirements and constraints to the future.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Co-developed-by: Mingwei Zhang <mizhang@google.com>
Signed-off-by: Mingwei Zhang <mizhang@google.com>
Tested-by: Xudong Hao <xudong.hao@intel.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Tested-by: Manali Shukla <manali.shukla@amd.com>
Link: https://patch.msgid.link/20251206001720.468579-17-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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