ArmPlatformPkg: Honour RES1 fields in CPTR_EL2HEADmaster

Unlike CPACR_EL1 whose reserved bits are solely RES0, CPTR_EL2 has some
RES1 bits, and so we should not clear them unless we know what they
mean. For example, when SVE was introduced, CPACR_EL1.ZEN occupied a
RES0 field and thus 0 means trap (which is what we get at EL1), but
CPTR_EL2.TZ occupied a RES1 field and thus 1 means trap, but we set it
to 0, so the environment is inconsistent between EDK2 and EL1 and EL2.
Another concrete case is for Morello, where the CEN/TC fields similarly
gate access to capability register state, but also alter exception
delivery and return, such that VBAR_ELx and ELR_ELx become capabilities.
So long as software adheres to RES0/1 this is backwards-compatible, but
since EDK2 does not do so here it inadvertently enables capability-based
exception delivery and return and thus, when run at EL2, gets stuck in a
trap loop when taking its first interrupt, but works just fine at EL1.

Fix this by setting all the RES1 fields in CPTR_EL2, following the
pattern for CPACR_EL1's non-zero initial value (due to setting FPEN so
as to not trap on SIMD/FP use), tested by running ArmVirtQemu-AARCH64
(DEBUG) on Morello QEMU with EL2 enabled.

Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>

0 files changed, 0 insertions, 0 deletions