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

Avi Kivity
2009-08-03T15:24:22Z

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();
}

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