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Date: Thu, 7 Jul 2016 13:15:54 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Wanpeng Li <kernellwp@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
        Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>,
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Subject: Re: [PATCH v2 0/4] implement vcpu preempted check
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On Thu, Jul 07, 2016 at 06:27:26PM +0800, Wanpeng Li wrote:
> In addition, I see xen's vcpu_runstate_info::state is updated during
> schedule, so I think I can do this similarly through kvm preemption
> notifier. IIUC, xen hypervisor has VCPUOP_get_runstate_info hypercall
> implemention, so the desired interface can be implemented if they add
> hypercall callsite in domU. I can add hypercall to kvm similarly.

So I suspect Xen has the page its writing to pinned in memory; so that a
write to it is guaranteed to not fault.

Otherwise I cannot see this working.

That is part of the larger surgery required for KVM steal time to get
'fixed'. Currently that steal time stuff uses kvm_write_guest_cached()
which appears to be able to fault in guest pages.

Or I'm not reading this stuff right; which is entirely possible.