[xen staging] libxl: Restore scheduling parameters after migrate in best-effort fashion

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[xen staging] libxl: Restore scheduling parameters after migrate in best-effort fashion

commit 162fc8295f31240dc3670190a91e9bbc03b0d7be
Author:     George Dunlap <[hidden email]>
AuthorDate: Wed Oct 10 12:36:25 2018 +0100
Commit:     George Dunlap <[hidden email]>
CommitDate: Wed Oct 10 12:36:25 2018 +0100

    libxl: Restore scheduling parameters after migrate in best-effort fashion
    Commit 3b4adba ("tools/libxl: include scheduler parameters in the
    output of xl list -l") added scheduling parameters to the set of
    information collected by libxl_retrieve_domain_configuration(), in
    order to report that information in `xl list -l`.
    Unfortunately, libxl_retrieve_domain_configuration() is also called by
    the migration / save code, and the results passed to the restore /
    receive code.  This meant scheduler parameters were inadvertently
    added to the migration stream, without proper consideration for how to
    handle corner cases.  The result was that if migrating from a host
    running one scheduler to a host running a different scheduler, the
    migration would fail with an error like the following:
    libxl: error: libxl_sched.c:232:sched_credit_domain_set: Domain 1:Getting domain sched credit: Invalid argument
    libxl: error: libxl_create.c:1275:domcreate_rebuild_done: Domain 1:cannot (re-)build domain: -3
    Luckily there's a fairly straightforward way to set parameters in a
    "best-effort" fashion.  libxl provides a single struct containing the
    parameters of all schedulers, as well as a parameter specifying which
    scheduler.  Parameters not used by a given scheduler are ignored.
    Additionally, the struct contains a parameter to specify the
    scheduler.  If you specify a specific scheduler,
    libxl_domain_sched_params_set() will fail if there's a different
    scheduler.  However, if you pass LIBXL_SCHEDULER_UNKNOWN, it will use
    the value of the current scheduler for that domain.
    In domcreate_stream_done(), before calling libxl__build_post(), set
    the scheduler to LIBXL_SCHEDULER_UNKNOWN.  This will propagate
    scheduler parameters from the previous instantiation on a best-effort
    Signed-off-by: George Dunlap <[hidden email]>
    Acked-by: Ian Jackson <[hidden email]>
    Acked-by: Wei Liu <[hidden email]>
 tools/libxl/libxl_create.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
index 320dbed3c6..4bb750e951 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -1218,6 +1218,15 @@ static void domcreate_stream_done(libxl__egc *egc,
         ret = ERROR_INVAL;
         goto out;
+    /*
+     * The scheduler on the sending domain may be different than the
+     * scheduler running here.  Setting the scheduler to UNKNOWN will
+     * cause the code to take to take whatever parameters are
+     * available in that scheduler, while discarding the rest.
+     */
+    info->sched_params.sched = LIBXL_SCHEDULER_UNKNOWN;
     ret = libxl__build_post(gc, domid, info, state, vments, localents);
     if (ret)
         goto out;
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