KVM: arm64: Explicitly handle BRBE traps as UNDEFINED

The Branch Record Buffer Extension (BRBE) adds a number of system registers
and instructions, which we don't currently intend to expose to guests. Our
existing logic handles this safely, but this could be improved with some
explicit handling of BRBE.

The presence of BRBE is currently hidden from guests as the cpufeature
code's ftr_id_aa64dfr0[] table doesn't have an entry for the BRBE field,
and so this will be zero in the sanitised value of ID_AA64DFR0 exposed to
guests via read_sanitised_id_aa64dfr0_el1(). As the ftr_id_aa64dfr0[] table
may gain an entry for the BRBE field in future, for robustness we should
explicitly mask out the BRBE field in read_sanitised_id_aa64dfr0_el1().

The BRBE system registers and instructions are currently trapped by the
existing configuration of the fine-grained traps. As neither the registers
are not described in the sys_reg_descs[] nor the instructions are described
in the sys_insn_descs[] table, emulate_sys_reg() will warn that these are
unknown before injecting an UNDEFINED exception into the guest.
Well-behaved guests shouldn't try to use the registers or instructions, but
badly-behaved guests could use these, resulting in unnecessary warnings. To
avoid those warnings, we should explicitly handle the BRBE registers, and
instructions as UNDEFINED.

Address the above by having read_sanitised_id_aa64dfr0_el1() mask out the
ID_AA64DFR0.BRBE field, and by adding sys_reg_descs entries for all of the
BRBE system registers, also by adding sys_insn_descs entries for all of the
BRBE instructions, treating these all as UNDEFINED.

Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: James Morse <james.morse@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: kvmarm@lists.linux.dev
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
----
Changes in V17:

- Updated the commit message including about sys_insn_descs[]
- Changed KVM to use existing SYS_BRBSRC/TGT/INF_EL1(n) format
- Moved the BRBE instructions into sys_insn_descs[] array