perf/x86/intel: Ensure LBRs are disabled when a CPU is starting

commit c631a2de upstream.

Explicitly clear DEBUGCTL.LBR when a CPU is starting, prior to purging the
LBR MSRs themselves, as at least one system has been found to transfer
control to the kernel with LBRs enabled (it's unclear whether it's a BIOS
flaw or a CPU goof).  Because the kernel preserves the original DEBUGCTL,
even when toggling LBRs, leaving DEBUGCTL.LBR as is results in running
with LBRs enabled at all times.

Closes: https://lore.kernel.org/all/c9d8269bff69f6359731d758e3b1135dedd7cc61.camel@redhat.com


Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20250131010721.470503-1-seanjc@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>