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    SubjectRe: [PATCH 3.16.y-ckt 17/17] Drivers: hv: vmbus: prevent cpu offlining on newer hypervisors
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    On Tue, 2016-04-12 at 19:08 +0100, Luis Henriques wrote:
    > 3.16.7-ckt27 -stable review patch.  If anyone has any objections, please let me know.
    >
    > ---8<------------------------------------------------------------
    >
    > From: Vitaly Kuznetsov <vkuznets@redhat.com>
    >
    > commit e513229b4c386e6c9f66298c13fde92f73e6e1ac upstream.
    >
    > When an SMP Hyper-V guest is running on top of 2012R2 Server and secondary
    > cpus are sent offline (with echo 0 > /sys/devices/system/cpu/cpu$cpu/online)
    > the system freeze is observed. This happens due to the fact that on newer
    > hypervisors (Win8, WS2012R2, ...) vmbus channel handlers are distributed
    > across all cpus (see init_vp_index() function in drivers/hv/channel_mgmt.c)
    > and on cpu offlining nobody reassigns them to CPU0. Prevent cpu offlining
    > when vmbus is loaded until the issue is fixed host-side.
    >
    > This patch also disables hibernation but it is OK as it is also broken (MCE
    > error is hit on resume). Suspend still works.
    [...]
    > +static void hv_cpu_hotplug_quirk(bool vmbus_loaded)
    > +{
    > + static void *previous_cpu_disable;
    > +
    > + /*
    > +  * Offlining a CPU when running on newer hypervisors (WS2012R2, Win8,
    > +  * ...) is not supported at this moment as channel interrupts are
    > +  * distributed across all of them.
    > +  */
    > +
    > + if ((vmbus_proto_version == VERSION_WS2008) ||
    > +     (vmbus_proto_version == VERSION_WIN7))
    > + return;
    > +
    > + if (vmbus_loaded) {
    > + previous_cpu_disable = smp_ops.cpu_disable;
    > + smp_ops.cpu_disable = hyperv_cpu_disable;
    > + pr_notice("CPU offlining is not supported by hypervisor\n");
    > + } else if (previous_cpu_disable)
    > + smp_ops.cpu_disable = previous_cpu_disable;
    [...]

    This is a really bad hack.  What if two different drivers patched
    smp_ops and got unloaded in a different order?  Perhaps the core
    support code for Hyper-V should define its own smp_ops.

    I don't want to stop this going into stable, but seriously, please
    clean this up.

    Ben.

    --
    Ben Hutchings
    In a hierarchy, every employee tends to rise to his level of incompetence.[unhandled content-type:application/pgp-signature]
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