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

[Avi Kivity <avi@redhat.com>]

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@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.
>
>    

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.


-- 
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