On Mon, 14 May 2007, Gregory Haskins wrote:
> >>> On Mon, May 14, 2007 at 1:23 PM, in message <[EMAIL PROTECTED]>,
> Avi Kivity <[EMAIL PROTECTED]> 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
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