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Subject: Re: [PATCH V2 3/5] ara virt interface of perf to support kvm guest
 os statistics collection in guest os
From: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
To: Gleb Natapov <gleb@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>, kvm@vger.kernel.org,
       Avi Kivity <avi@redhat.com>, Ingo Molnar <mingo@elte.hu>,
       Fr??d??ric Weisbecker <fweisbec@gmail.com>,
       Arnaldo Carvalho de Melo <acme@redhat.com>,
       Cyrill Gorcunov <gorcunov@gmail.com>, Lin Ming <ming.m.lin@intel.com>,
       Sheng Yang <sheng@linux.intel.com>,
       Marcelo Tosatti <mtosatti@redhat.com>, oerg Roedel <joro@8bytes.org>,
       Jes Sorensen <Jes.Sorensen@redhat.com>,
       Zachary Amsden <zamsden@redhat.com>, zhiteng.huang@intel.com,
       tim.c.chen@intel.com, Peter Zijlstra <a.p.zijlstra@chello.nl>
In-Reply-To: <20100621135659.GL4689@redhat.com>
References: <1277112703.2096.511.camel@ymzhang.sh.intel.com>
	 <20100621135659.GL4689@redhat.com>
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Date: Tue, 22 Jun 2010 13:47:16 +0800
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On Mon, 2010-06-21 at 16:56 +0300, Gleb Natapov wrote:
> On Mon, Jun 21, 2010 at 05:31:43PM +0800, Zhang, Yanmin wrote:
> > The 3rd patch is to implement para virt perf at host kernel.
> > 
> > Signed-off-by: Zhang Yanmin <yanmin_zhang@linux.intel.com>
> > 
<snip>

> > +
> > +static void kvm_copy_event_to_guest(struct kvm_vcpu *vcpu,
> > +			struct perf_event *host_event)
> > +{
> > +	struct host_perf_shadow *shadow = host_event->host_perf_shadow;
> > +	struct guest_perf_event counter;
> > +	int ret;
> > +	s32 overflows;
> > +
> > +	ret = kvm_read_guest(vcpu->kvm, shadow->guest_event_addr,
> > +				&counter, sizeof(counter));
> > +	if (ret < 0)
> > +		return;
> > +
> > +again:
> > +	overflows = atomic_read(&shadow->counter.overflows);
> > +	if (atomic_cmpxchg(&shadow->counter.overflows, overflows, 0) !=
> > +			overflows)
> > +		goto again;
> > +
> > +	counter.count = shadow->counter.count;
> > +	atomic_add(overflows, &counter.overflows);
> > +
> > +	kvm_write_guest(vcpu->kvm,
> > +			shadow->guest_event_addr,
> > +			&counter,
> > +			sizeof(counter));
> Those kind of interfaces worry me since the can cause bugs that are
> very hard to catch.  What if guest enables some events and crashes into
> kdump kernel (or kexec new kernel) without reseting HW. Now host may
> write over guest memory without guest expecting it. Do you handle this
> scenario in a guest side? I think you need to register reboot notify
> and disable events from there.
Sorry for missing your comments.

My patch could take care of dead guest os by cleaning up all events in function
kvm_arch_destroy_vm, so all events are closed if host user kills the guest
qemu process.

As for your scenario, I will register reboot notify and add a new pv perf
hypercall interface to vmexit to host kernel to do cleanup.

Thanks,
Yanmin


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