linux-kernel - Re: [PATCH-RFC 2/2] eventfd: EFD

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Message-ID: <4A770260.5000507@redhat.com>
Date:	Mon, 03 Aug 2009 18:29:36 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	davidel@...ilserver.org, gleb@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH-RFC 2/2] eventfd: EFD_STATE flag

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

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