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

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Date:	Thu, 7 Jul 2016 18:12:51 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
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

2016-07-07 17:42 GMT+08:00 Peter Zijlstra <peterz@...radead.org>:
> On Thu, Jul 07, 2016 at 04:48:05PM +0800, Wanpeng Li wrote:
>> 2016-07-06 20:28 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>> > Hmm, you're right.  We can use bit 0 of struct kvm_steal_time's flags to
>> > indicate that pad[0] is a "VCPU preempted" field; if pad[0] is 1, the
>> > VCPU has been scheduled out since the last time the guest reset the bit.
>> >  The guest can use an xchg to test-and-clear it.  The bit can be
>> > accessed at any time, independent of the version field.
>>
>> If one vCPU is preempted, and guest check it several times before this
>> vCPU is scheded in, then the first time we can get "vCPU is
>> preempted", however, since the field is cleared, the second time we
>> will get "vCPU is running".
>>
>> Do you mean we should call record_steal_time() in both kvm_sched_in()
>> and kvm_sched_out() to record this field? Btw, if we should keep both
>> vcpu->preempted and kvm_steal_time's "vCPU preempted" field present
>> simultaneous?
>
> I suspect you want something like so; except this has holes in.
>
> We clear KVM_ST_PAD_PREEMPT before disabling preemption and we set it
> after enabling it, this means that if we get preempted in between, the
> vcpu is reported as running even though it very much is not.

Paolo also point out this to me offline yesterday: "Please change
pad[12] to "__u32 preempted; __u32 pad[11];" too, and remember to
update Documentation/virtual/kvm/msr.txt!". Btw, do this in preemption
notifier means that the vCPU is real preempted on host, however,
depends on vmexit is different semantic I think.

Regards,
Wanpeng Li