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    SubjectRe: [PATCH v2 0/3] Freezer, CPU hotplug, x86 Microcode: Fix task freezing failures
    On 10/11/2011 12:23 AM, tj@kernel.org wrote:
    > Hello,
    >
    > On Mon, Oct 10, 2011 at 08:34:43PM +0200, Borislav Petkov wrote:
    >> On Mon, Oct 10, 2011 at 02:08:48PM -0400, tj@kernel.org wrote:
    >>> Maybe I'm confused but is that patch correct for actual CPU hotplug
    >>> case? If not, what's the point in doing that? What are we gonna do
    >>> after six month some people come up with "CPU hotplug fails to load
    >>> new microcode for the new CPU"?
    >>
    >> Ok, first of all, we still will load ucode on the onlining path - we're
    >> simply not going to reload it when the CPU has gone offline and onlined
    >> again. For that case people should simply reload the module so that
    >> ucode on _all_ CPUs is updated pretty much at same time.
    >
    > I was thinking about hot-swap. It might be pretty unlikely at this
    > point but I don't think excluding that is a good idea. x86 is used in
    > pretty highend too these days. Again, I don't know much about how
    > ucodes are supposed to be managed and maybe it's true that we don't
    > need new one at all even after hotswap. If that's the case, state it
    > clearly and it's all fine.
    >
    >>> The invalidation code is there for a reason.
    >>
    >> ... and that reason being?
    >
    > Again, the CPU for the microcode is going away? It's something tied
    > to a device and the device is going away. It's a basic correctness
    > issue. It at least needs to be revalidated.
    >
    >>> If somebody is sure that microcode don't need to be changed once
    >>> loaded, then all's good and dandy but that's not the case here, right?
    >>
    >> Well, basically the current situation didn't change the ucode - it
    >> simply reloaded the same image from before going offline.
    >>
    >> See, there's this another problem with what we have right now: imagine
    >> you've just updated the ucode image on disk and offline only a subset of
    >> the cores. Then you online them again and they now get the newer ucode
    >> image while the others still run the old ucode. This could explode or
    >> could not, one thing's for sure: all bets are off. If we don't reload it
    >> on hotplug, we're fine - only module reload triggers the ucode update in
    >> a fairly synchronized manner.
    >
    > Yeah, loading different ucodes to different cores sounds pretty scary.
    > I suppose we'll need to distinguish physical hotplugs from logical
    > ones.
    >
    > Hmm... is it possible to tell whether the core coming online is the
    > same one as the last time? If that's possible, the problem becomes
    > pretty simple and we can simply tell people who are mixing
    > suspend/hibernate with physical hotplug that they're crazy.
    >

    I think that is pretty easy, atleast from a microcode revision standpoint:
    the collect_cpu_info() function (defined in arch/x86/kernel/microcode_core.c
    and arch/x86/kernel/microcode_intel.c or ..._amd.c) can be used for that
    purpose. Am I right Boris?

    --
    Regards,
    Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    Linux Technology Center,
    IBM India Systems and Technology Lab



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