linux-kernel - Re: [PATCH 0/2] eventfd: new EFD

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Message-ID: <alpine.DEB.2.00.0908261242530.7780@makko.or.mcafeemobile.com>
Date:	Wed, 26 Aug 2009 12:44:10 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Avi Kivity <avi@...hat.com>
cc:	"Michael S. Tsirkin" <mst@...hat.com>, gleb@...hat.com,
	kvm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] eventfd: new EFD_STATE flag

On Wed, 26 Aug 2009, Avi Kivity wrote:

> On 08/26/2009 10:13 PM, Davide Libenzi wrote:
> > Ok, so why not using the eventfd counter as state?
> > On the device side:
> > 
> > void write_state(int sfd, int state) {
> > 	u64 cnt;
> > 
> > 	/* Clear the current state, sfd is in non-blocking mode */
> > 	read(sfd,&cnt, sizeof(cnt));
> > 	/* Writes new state */
> > 	cnt = 1 + !!state;
> > 	write(sfd,&cnt, sizeof(cnt));
> > }
> > 
> > 
> > On the hypervisor side:
> > 
> > int read_state(int sfd) {
> > 	u64 cnt;
> > 
> > 	read(sfd,&cnt, sizeof(cnt));
> > 	return state - 1;
> > }
> > 
> >    
> 
> Hadn't though of read+write as set.  While the 1+ is a little ugly, it's
> workable.

Pick what you want, as long as it always writes something != 0 :)


> I see no kernel equivalent to read(), but that's easily done.

Adding an in-kernel read based on "ctx", that is no problem at all.



- Davide


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