Red Hat Bugzilla – Bug 996692
virtio-serial: write() call does not always block when host is not connected
Last modified: 2015-02-17 11:45:05 EST
Created attachment 786228 [details]
host.c and guest.c
Description of problem:
The Guest API for virtio-serial (http://www.linux-kvm.org/page/Virtio-serial_API) indicates that a write(2) call will block if no host app is connected to the socket. However, under certain conditions, this does not happen. If a host app was INTerrupted and subsequently terminated during a read() call, the following write() call in the guest app will succeed.
Version-Release number of selected component (if applicable):
Fedora 19 x86_64 with qemu-kvm-1.4.2-4
Steps to Reproduce:
The steps assume your VM is configured with a virtio-serial port named foobar.
1. Compile host.c and run on host machine, it will block on read()
2. Interrupt (^C) the application to terminate it
3. Compile guest.c and run on guest machine
The write() call in the guest app succeeds.
The write() call in the guest app blocks because no host app is connected to the socket.
qemu doesn't get the notification that a host-side process is terminated (and hence the socket is disconnected). This is a limitation of the qemu's chardev layer, and a difficult one to fix in general. This is a known problem, but we can't say when it'll be resolved.
BTW subsequent guest writes will block.
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