On Mon, Aug 03, 2009 at 06:29:36PM +0300, Avi Kivity wrote: > On 08/03/2009 06:14 PM, Michael S. Tsirkin wrote: >> On Mon, Aug 03, 2009 at 06:09:38PM +0300, Avi Kivity wrote: >> >>> On 07/28/2009 08:55 PM, Michael S. Tsirkin wrote: >>> >>>> This implements a new EFD_STATE flag for eventfd. >>>> When set, this flag changes eventfd behaviour in the following way: >>>> - write simply stores the value written, and is always non-blocking >>>> - read unblocks when the value written changes, and >>>> returns the value written >>>> >>>> Motivation: we'd like to use eventfd in qemu to pass interrupts from >>>> (emulated or assigned) devices to guest. For level interrupts, the >>>> counter supported currently by eventfd is not a good match: we really >>>> need to set interrupt to a level, typically 0 or 1, and give the guest >>>> ability to see the last value written. >>>> >>>> >>>> @@ -31,37 +31,59 @@ struct eventfd_ctx { >>>> * issue a wakeup. >>>> */ >>>> __u64 count; >>>> + /* >>>> + * When EF_STATE flag is set, eventfd behaves differently: >>>> + * value written gets stored in "count", read will copy >>>> + * "count" to "state". >>>> + */ >>>> + __u64 state; >>>> unsigned int flags; >>>> }; >>>> >>>> >>> Why not write the new value into ->count directly? >>> >> >> That's what it says. state is ther to detect that value was changed >> after last read. Makes sense? >> > > Why not do it at the point of the write? > > if (value != ctx->count) { > ctx->count = value; > wake_things_up(); > }
What if write comes before read? > -- > error compiling committee.c: too many arguments to function -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html