[Xen-devel] Issues with the xen-access.c model

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

Hello,

xen-access.c does roughly this:

for (;;) {
        poll_evt_channel();

        if (new_events) {
                while (ring_buffer_has_requests) {
                        pull_request();
                        process_request();
                        put_response();
                }

                signal_evt_channel();
        }
}

The problems are these:

1. vm_event_put_request() does notify_via_xen_event_channel(d,
ved->xen_port); at the very end, and we seem to be using a FIFO event
channel (if I'm reading the code right, it's generic event channel code
that pre-dates the vm_event code that sits on top of it).

This means that for a guest that's running two VCPUs, the following
scenario is possible:

* VCPU1 puts an event in the ring buffer
* VCPU1 signals the event channel
* poll_evt_channel() wakes up
* VCPU2 puts an event in the ring buffer
* the loop processes the first event
* VCPU2 signals the event channel
* the loop processes the second event (since there _are_ more requests
in the ring buffer)
* the loop signals the event channel that requests have been processed
* poll_evt_channel() wakes up again, because of the second event channel
signal, but this is pointless since all events have already been processed

This can be avoided by counting the requests processed vs. the number of
succesful poll()s, but it still does not solve the second problem:

2. Race conditions. At least in theory, there's nothing to stop the
second iteration of the request processing loop to read a partially
written request from the ring buffer, or garbage left over from a
previous request, if it hits it "just right" (i.e. the "VCPU2 puts an
event in the ring buffer" part can run on top of the "the loop processes
the second event").

All this doesn't really show up unless we're in heavy processing
scenarios, but they can occur, especially in synced vm_event scenarios.

The problems seems to be alleviated by changing the loop to this:

for (;;) {
        poll_evt_channel();

        if (new_events) {
                while (ring_buffer_has_requests) {
                        pull_request();
                        process_request();
                        put_response();

                        /* Moved here. */
                        signal_evt_channel();
                }
        }
}

but it's obviously not foolproof, the main problem remaining the
synchronization of the ring buffer from both the hypervisor side and
userspace.

How should we proceed to fix this?


Thanks,
Razvan

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