Re: [PATCH-RFC 2/2] eventfd: EFD_STATE flag

Michael S. Tsirkin
2009-08-03T16:58:21Z

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