Re: [PATCH 5/9] KVM: Adds ability to signal userspace using a file-descriptor

["Gregory Haskins" <ghaskins-Et1tbQHTxzrQT0dZR+AlfA@public.gmane.org>]

>>> On Tue, May 15, 2007 at  3:45 AM, in message <4649650E.2070102-atKUWr5tajBWk0Htik3J/w@public.gmane.org>,
Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote: 
> Gregory Haskins wrote:
>> Signed- off- by: Gregory Haskins <ghaskins-Et1tbQHTxzrQT0dZR+AlfA@public.gmane.org>
>> ---
>>
>>  drivers/kvm/kvm.h      |    1 +
>>  drivers/kvm/kvm_main.c |   52 ++++++++++++++++++++++++++++++++++++++++++------
>>  include/linux/kvm.h    |    1 +
>>  3 files changed, 48 insertions(+), 6 deletions(- )
>>
>> diff -- git a/drivers/kvm/kvm.h b/drivers/kvm/kvm.h
>> index 7b5d5e6..f5731c4 100644
>> ---  a/drivers/kvm/kvm.h
>> +++ b/drivers/kvm/kvm.h
>> @@ - 333,6 +333,7 @@ struct kvm_vcpu_irq {
>>  	int                  deferred;
>>  	struct task_struct  *task;
>>  	int                  guest_mode;
>> +	int                  eventfd;
>>   
> 
> Best to convert the fd to a filp when you install it.  This avoids the
> conversion during runtime and allows you to do error checking earlier.


That was my initial impression also, but then I realized there was a problem with that:  Eventfd doesnt appear to have any way to notify other entities when the fd is closed.  Therefore the filp could be left dangling in this case.  By using the fd instead, I can validate the pointer each time I need it.  Perhaps Davide will have a suggestion here.

Regardless, I do like the idea of checking the fd on assignment.  I will put an eventfd_fget() check into the ioctl too just to catch it early.  I might also add logic to invalidate irq.eventfd if the eventfd_fget() ever fails at runtime because userspace neglects to explicitly unregister (so we don't keep checking an invalid fd over and over).

-Greg

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