Zero out the LBR capabilities during PMU refresh to avoid exposing LBRs
to the guest against userspace's wishes. If userspace modifies the
guest's CPUID model or invokes KVM_CAP_PMU_CAPABILITY to disable vPMU
after an initial KVM_SET_CPUID2, but before the first KVM_RUN, KVM will
retain the previous LBR info due to bailing before refreshing the LBR
descriptor.
Note, this is a very theoretical bug, there is no known use case where a
VMM would deliberately enable the vPMU via KVM_SET_CPUID2, and then later
disable the vPMU.
Link: https://lore.kernel.org/r/20230311004618.920745-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
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