arm64: Restore trapless ptimer access

arm64: Restore trapless ptimer access

Due to recent KVM changes, QEMU is setting a ptimer offset resulting
in unintended trap and emulate access and a consequent performance
hit. Filter out the PTIMER_CNT register to restore trapless ptimer
access.

Quoting Andrew Jones:

Simply reading the CNT register and writing back the same value is
enough to set an offset, since the timer will have certainly moved
past whatever value was read by the time it's written.  QEMU
frequently saves and restores all registers in the get-reg-list array,
unless they've been explicitly filtered out (with Linux commit
680232a94c12, KVM_REG_ARM_PTIMER_CNT is now in the array). So, to
restore trapless ptimer accesses, we need a QEMU patch to filter out
the register.

See
https://lore.kernel.org/kvmarm/gsntttsonus5.fsf@coltonlewis-kvm.c.googlers.com/T/#m0770023762a821db2a3f0dd0a7dc6aa54e0d0da9
for additional context.

Cc: qemu-stable@nongnu.org
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>
Signed-off-by: Colton Lewis <coltonlewis@google.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Colton Lewis <coltonlewis@google.com>
Message-id: 20230831190052.129045-1-coltonlewis@google.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>