Right now the value is exposed to userspace via GET_VRING_BASE, so only last_avail_idx is synced. If we need sync last_used_idx, we should also sync pending indices which requires more thoughts.

Technically it doesn't sound right - crossing the boundary a bit even with simplified form of assumption. But depending on how userspace could make use of this API, it doesn't seem it breaks existing functionality for the moment.

I don't get here, maybe you can explain a little bit more?