[Xen-devel] A race condition introduced by changeset 15175: Re-init hypercall stubs page after HVM save/restore

["Cui, Dexuan" <dexuan.cui@xxxxxxxxx>]

For an SMP Linux HVM guest with PV drivers inserted, when we do save/restore 
(or LiveMigration) for the guest, it might panic after it's restored.
The panic point is inside ap_suspend():
 ....
    while (info->do_spin) {
        cpu_relax();
        read_lock(&suspend_lock);
        HYPERVISOR_yield();      ----> guest might panic on the invocation of 
this function.
        read_unlock(&suspend_lock);
    }
...

The root cause is: ap might be invoking the hypercall while bsp is asking the 
hypervisor to re-initialize the hypercall page when the guest has been just 
restored!

What's the purpose of re-initializing the hypercall page here? To improve the 
compatibility in the case the src/target hosts have different hypercall stub 
codes?

PS, I'm using c/s 18353 to debug the issue. (the latest xen-unstable.hg's S/R 
and L/M are broken by 18383.

Thanks,
-- Dexuan

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