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[Xen-changelog] [xen master] libxc: Document xenctrl.h event channel calls

commit 8a91554484ad6977f641b308af38f337c20e97cc
Author:     Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
AuthorDate: Tue Dec 17 18:35:18 2013 +0000
Commit:     Ian Campbell <ian.campbell@xxxxxxxxxx>
CommitDate: Tue Jan 7 13:31:20 2014 +0000

    libxc: Document xenctrl.h event channel calls
    Provide semantic documentation for how the libxc calls relate to the
    hypervisor interface, and how they are to be used.
    Also document the bug (present at least in Linux 3.12) that setting
    the evtchn fd to nonblocking doesn't in fact make xc_evtchn_pending
    nonblocking, and describe the appropriate workaround.
    Signed-off-by: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
    CC: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
    Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
    CC: Jan Beulich <JBeulich@xxxxxxxx>
    CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
 tools/libxc/xenctrl.h |   27 ++++++++++++++++++++++++++-
 1 files changed, 26 insertions(+), 1 deletions(-)

diff --git a/tools/libxc/xenctrl.h b/tools/libxc/xenctrl.h
index 096a590..13f816b 100644
--- a/tools/libxc/xenctrl.h
+++ b/tools/libxc/xenctrl.h
@@ -1043,6 +1043,18 @@ int xc_evtchn_close(xc_evtchn *xce);
  * Return an fd that can be select()ed on.
+ *
+ * Note that due to bugs, setting this fd to non blocking may not
+ * work: you would hope that it would result in xc_evtchn_pending
+ * failing with EWOULDBLOCK if there are no events signaled, but in
+ * fact it may block.  (Bug is present in at least Linux 3.12, and
+ * perhaps on other platforms or later version.)
+ *
+ * To be safe, you must use poll() or select() before each call to
+ * xc_evtchn_pending.  If you have multiple threads (or processes)
+ * sharing a single xce handle this will not work, and there is no
+ * straightforward workaround.  Please design your program some other
+ * way.
 int xc_evtchn_fd(xc_evtchn *xce);
@@ -1082,7 +1094,20 @@ int xc_evtchn_unbind(xc_evtchn *xce, evtchn_port_t port);
  * Return the next event channel to become pending, or -1 on failure, in which
- * case errno will be set appropriately.  
+ * case errno will be set appropriately.
+ *
+ * At the hypervisor level the event channel will have been masked,
+ * and then cleared, by the underlying machinery (evtchn kernel
+ * driver, or equivalent).  So if the event channel is signaled again
+ * after it is returned here, it will be queued up, and delivered
+ * again after you unmask it.  (See the documentation in the Xen
+ * public header event_channel.h.)
+ *
+ * On receiving the notification from xc_evtchn_pending, you should
+ * normally: check (by other means) what work needs doing; do the
+ * necessary work (if any); unmask the event channel with
+ * xc_evtchn_unmask (if you want to receive any further
+ * notifications).
 xc_evtchn_pending(xc_evtchn *xce);
generated by git-patchbot for /home/xen/git/xen.git#master

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