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    SubjectRe: [PATCH v2] x86/power: Fix 'nosmt' vs. hibernation triple fault during resume
    On Wed, May 29, 2019 at 12:32:02PM +0200, Jiri Kosina wrote:
    > From: Jiri Kosina <jkosina@suse.cz>
    >
    > As explained in
    >
    > 0cc3cd21657b ("cpu/hotplug: Boot HT siblings at least once")
    >
    > we always, no matter what, have to bring up x86 HT siblings during boot at
    > least once in order to avoid first MCE bringing the system to its knees.
    >
    > That means that whenever 'nosmt' is supplied on the kernel command-line,
    > all the HT siblings are as a result sitting in mwait or cpudile after
    > going through the online-offline cycle at least once.
    >
    > This causes a serious issue though when a kernel, which saw 'nosmt' on its
    > commandline, is going to perform resume from hibernation: if the resume
    > from the hibernated image is successful, cr3 is flipped in order to point
    > to the address space of the kernel that is being resumed, which in turn
    > means that all the HT siblings are all of a sudden mwaiting on address
    > which is no longer valid.
    >
    > That results in triple fault shortly after cr3 is switched, and machine
    > reboots.
    >
    > Fix this by always waking up all the SMT siblings before initiating the
    > 'restore from hibernation' process; this guarantees that all the HT
    > siblings will be properly carried over to the resumed kernel waiting in
    > resume_play_dead(), and acted upon accordingly afterwards, based on the
    > target kernel configuration.

    hibernation_restore() is called by user space at runtime, via ioctl or
    sysfs. So I think this still doesn't fix the case where you've disabled
    CPUs at runtime via sysfs, and then resumed from hibernation. Or are we
    declaring that this is not a supported scenario?

    Would it be possible for mwait_play_dead() to instead just monitor a
    fixmap address which doesn't change for kaslr?

    Is there are reason why maxcpus= doesn't do the CR4.MCE booted_once
    dance?

    --
    Josh

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