KVM: SVM: Drop DEBUGCTL[5:2] from guest's effective value

Drop bits 5:2 from the guest's effective DEBUGCTL value, as AMD changed
the architectural behavior of the bits and broke backwards compatibility.
On CPUs without BusLockTrap (or at least, in APMs from before ~2023),
bits 5:2 controlled the behavior of external pins:

  Performance-Monitoring/Breakpoint Pin-Control (PBi)—Bits 5:2, read/write.
  Software uses thesebits to control the type of information reported by
  the four external performance-monitoring/breakpoint pins on the
  processor. When a PBi bit is cleared to 0, the corresponding external pin
  (BPi) reports performance-monitor information. When a PBi bit is set to
  1, the corresponding external pin (BPi) reports breakpoint information.

With the introduction of BusLockTrap, presumably to be compatible with
Intel CPUs, AMD redefined bit 2 to be BLCKDB:

  Bus Lock #DB Trap (BLCKDB)—Bit 2, read/write. Software sets this bit to
  enable generation of a #DB trap following successful execution of a bus
  lock when CPL is > 0.

and redefined bits 5:3 (and bit 6) as "6:3 Reserved MBZ".

Ideally, KVM would treat bits 5:2 as reserved.  Defer that change to a
feature cleanup to avoid breaking existing guest in LTS kernels.  For now,
drop the bits to retain backwards compatibility (of a sort).

Note, dropping bits 5:2 is still a guest-visible change, e.g. if the guest
is enabling LBRs *and* the legacy PBi bits, then the state of the PBi bits
is visible to the guest, whereas now the guest will always see '0'.

Reported-by: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: stable@vger.kernel.org
Reviewed-and-tested-by: Ravi Bangoria <ravi.bangoria@amd.com>
Link: https://lore.kernel.org/r/20250227222411.3490595-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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