Unfortunately I found one issue: if PF is down, and then the VF on the guest is
down as well (ip link down) and then goes back up before the PF, then calling
rte_eth_dev_reset will return 0 (success), even though the PF is still down and it
should fail. This is with ixgbe. Any idea what could be the problem?

I've found this interesting thing. I believe it’s the HW difference between igb and ixgbe. When the link is down, ixgbe VF can be reset successfully but igb VF cannot. The expression is the  registers of the ixgbe VF can be accessed when the PF link is down but igb VF cannot.
It means, on ixgbe, when PF link is down, we reset the VF link. Then PF link is up, we receive the message again and reset the VF link again. 
But on igb, when PF link is down, we cannot reset VF link successfully, so when the PF link is up, we cannot receive the message. No trigger for us to reset the VF link again. That's why on igb we have to try again and again until it succeed, means until PF link is up.
So the return 0 by rte_eth_dev_reset means the resetting succeeded, not mean the rx/tx is ready. Rx/tx has to depend on the PF link is up.

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Kind regards,
Luca Boccassi