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

[Michael S. Tsirkin]

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?

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