Re: [PATCH v3 4/7] x86/hyper-v: allocate and use Virtual Processor Assist Pages

[Thomas Gleixner]

On Thu, 15 Mar 2018, Peter Zijlstra wrote:
> On Thu, Mar 15, 2018 at 12:45:03PM +0100, Thomas Gleixner wrote:
> > On Thu, 15 Mar 2018, Vitaly Kuznetsov wrote:
> > > The only user of these pages is currently KVM. Can we still have vCPUs
> > > running on the outgoing CPU at this point? If case we can we're in
> > > trouble and we need to somehow kick them out first.
> > 
> > The first thing we do in unplug is to mark the CPU inactive, but I'm not
> > sure whether that prevents something which was on the CPU before and
> > perhaps preempted or is affine to that CPU to be scheduled in
> > again. Peter????
> 
> I think we can still have tasks running at this point.
> 
> AP_ACTIVE (sched_cpu_deactivate) simply takes the CPU out of the active
> mask, which guarantees no new tasks will land on the CPU.
> 
> We'll then proceed all the way to TEARDOWN_CPU as 'normal', at which
> point we'll call stop_machine() which does the old DYING callbacks.
> 
> It sounds like we want this done here, although possibly we can't do
> vfree() from that context, in which case it needs to store the pointer
> and do that from a BP callback (what used to be the OFFLINE callbacks or
> something).

So the wrmsr() wants to be in the dying range. The vfree() is questionable
anyway because the re-onlining of that CPU will just allocate it again. So
it could very well stay around.

Thanks,

	tglx