linux-kernel - Re: [PATCH v2 0/4] implement vcpu preempted check

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Date:	Thu, 7 Jul 2016 13:15:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
	linux-s390 <linux-s390@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>, mpe@...erman.id.au,
	boqun.feng@...il.com, will.deacon@....com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Waiman Long <waiman.long@....com>,
	virtualization@...ts.linux-foundation.org,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>, benh@...nel.crashing.org,
	schwidefsky@...ibm.com, Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 0/4] implement vcpu preempted check

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.