Re: [PATCH 05/10] KVM: Adds ability to signal userspace using a file-descriptor

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

>>> On Mon, May 14, 2007 at  3:12 PM, in message
<Pine.LNX.4.64.0705141207070.19682-GPJ85BhbkB8RepQJljzAVbITYcZ0+W3JAL8bYrjMMd8@public.gmane.org>, Davide Libenzi
<davidel-AhlLAIvw+VEjIGhXcJzhZg@public.gmane.org> wrote: 
> On Mon, 14 May 2007, Gregory Haskins wrote:
> 
>> >>> On Mon, May 14, 2007 at  1:23 PM, in message <46489B12.8030807-atKUWr5tajBWk0Htik3J/w@public.gmane.org>,
>> Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote: 
>> >
>> >> Do you close the signaled fd after receiving the signal/event? If you 
>> >> don't close it, eventfd will always return ready (POLLIN).
>> >>   
>> > 
>> > We don't.  Anyway, that's what we thought.  Thanks for the confirmation.
>> > 
>> 
>> And plus I just finished converting to Davide's eventfd, so its moot ;)
>> 
>> On that topic, I could use some advice:
>> 
>> I was originally planning on adding a new ioctl like KVM_VCPU_CREATE_EVENTFD 
> which would allocate a new eventfd and return it.  However, I soon realized 
> that the only method to create an eventfd is sys_eventfd(), which is not 
> exported by the eventfd.h headerfile (presumably this must be a new system 
> call).
>> 
>> So based on that, I figured I would change the model so that the usermode 
> app should call the eventfd open() call on its own, and then they could 
> register the fd with me.  So KVM_VCPU_CREATE_EVENTFD becomes 
> KVM_VCPU_SET_EVENTFD (where - 1 "unregisters" it).
>> 
>> Does this sound like a reasonable approach?  If so, how does the usermode 
> app actually open the eventfd today?  Is there a new glibc that I need to get 
> the new system call?  Or can the app use open() somehow?  If open(), what is 
> the path that should be specified?
>> 
>> Conversely, if my first approach was the right one how do I invoke the 
> sys_eventfd()?  Is there a way to invoke system calls in kernel mode?  A 
> better way?
>> 
>> Any advice appreciated.
> 
> The eventfd syscall is defined in include/linux/syscalls.h
> From userspace, till glibc aligns:
> 
> #include <sys/syscall.h>
> 
> #ifndef __NR_eventfd
> #if defined(__x86_64__)
> #define __NR_eventfd 283
> #elif defined(__i386__)
> #define __NR_eventfd 323
> #else
> #error Cannot detect your architecture!
> #endif
> #endif
> 
> static int eventfd(int count) {
> 
>         return syscall(__NR_eventfd, count);
> }
> 
> 
> If the kernel side receives an fd from userspace, it must use:
> 
> file = eventfd_fget(fd);
> if (IS_ERR(file))
>     ....
> eventfd_signal(file, 1);
> fput(file);
> 
> 
> 
> -  Davide

Thanks Davide,
   That is very helpful.  Is there any reason why we can't export eventfd_signal() and eventfd_fget() to modules?

-Greg

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