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

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Davide Libenzi wrote:
> On Tue, 15 May 2007, Gregory Haskins wrote:
>
>   
>>>>> 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.
>>     
>
> I don't know how critical is the path where you will be doing check. The 
> eventfd_fget() is pretty fast, so if you're not looking at a performance 
> critical path, I'd suggest that. Otherwise you can do an early 
> eventfd_get, and keep the file*. If you have no the ways to know if the 
> userspace disconnected, an atomic_read(&file->f_count)==1 will tell you 
> that you're the only owner of the file* (that is, userspace closed the 
> eventfd descriptor). I'd give preference to the former option though.
>   

We aren't really interested whether userspace is looking or not.  After 
all, it installed the eventfd, so it must be interested in the events.

> Avi, as you may have read from lkml, Andrew prefers the eventfd symbols 
> export to come through the kvm tree, since they do not want to export 
> symbols that so far has no module users.
>
>   

Yes, I'll pick up the patch from lkml.


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


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