[Xen-users] Scheduling questions for I/O activity

["Amitayu Das" <amitayudas@xxxxxxxxx>]

Hi,

Had two more questions about scheduling mechanisms employed by Xen.
Can you please answer them?

1. Say, DomU1 started transmitting packets over the network and got
blocked. Dom0 is supposed to do the actual transmission and it did
that. Eventually, when the reply came from the other side, nither
Dom0 nor DomU1 is scheduled. How does hypervisor react to this
interrupt? I can think of the following situations and you may be
able to tell me which is correct:

a. hypervisor preempts the currently running entity (which is
neither DomU1/Dom0) and schedules Dom0 to complete the I/O. When
Dom0 is done,
the preempted guy is scheduled again.

b. hypervisor preempts the currently running entity (which is
neither
DomU1/Dom0) and schedules Dom0 to complete the I/O. When Dom0 is
done,
DomU1 is scheduled.

c. hypervisor does not do any preemption; schedules Dom0/DomU1
according to the predefined order.

I don't know if (c) is actually possible or not. If anything else
happens in reality, please let me know.

2. Unmodified guest OSs can run on VT-x machine. What happens to the I/O activity
done by each guest OS? Do they still rely on Dom0 or they perform
I/O by themselves?

Thanks,
Amitayu
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