Trustzone: DSB before/after SMC

[Mason]

Hello,

I have a few questions about SMC. (I'm using Cortex-A9)

Platforms that use SMC often/always execute DSB beforehand.

1a) Is DSB required before SMC?
1b) Is DSB required  after SMC?
2a) Is DSB required before returning to non-secure OS?
2b) Is DSB required  after returning to non-secure OS?
3) Is this documented in the ARM ARM?

Bonus question: if DSB is required before SMC, why didn't ARM
make SMC implicitly do a DSB?

Regards.