Re: [PATCH V2 3/5] ara virt interface of perf to support kvm guestos statistics collection in guest os

[Avi Kivity]

On 06/22/2010 08:47 AM, Zhang, Yanmin wrote:
> 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@xxxxxxxxxxxxxxx>
> > >
> > >        
> <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.
>
>    

A reset does not involve destroying a vm; you have to clean up as part 
of the rest process.

Note MSRs are automatically cleared, so that's something in favour of an 
MSR interface.

> 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.
>    

You aren't guaranteed a reboot notifier will be called.  On the other 
hand, we need a kexec handler.


--
error compiling committee.c: too many arguments to function

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