FROMLIST: arm64: kvm: stop treating register x18 as caller save

In preparation of reserving x18, stop treating it as caller save in
the KVM guest entry/exit code. Currently, the code assumes there is
no need to preserve it for the host, given that it would have been
assumed clobbered anyway by the function call to __guest_enter().
Instead, preserve its value and restore it upon return.

Bug: 145210207
Change-Id: I12ef981133176a5092b091a060c8d2493cb6f0c3
(am from https://lore.kernel.org/patchwork/patch/1149065/)
Link: https://patchwork.kernel.org/patch/9836891/


Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
[Sami: updated commit message, switched from x18 to x29 for the guest context]
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>