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

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Message-ID: <20090803165708.GB3630@redhat.com>
Date:	Mon, 3 Aug 2009 19:57:08 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Avi Kivity <avi@...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 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
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