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

[Avi Kivity]

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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