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

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Message-ID: <20090826195002.GE11762@redhat.com>
Date:	Wed, 26 Aug 2009 22:50:02 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Davide Libenzi <davidel@...ilserver.org>,
	"Michael S. Tsirkin" <mst@...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, Aug 26, 2009 at 10:42:05PM +0300, 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.
> 
It's two system calls instead of one to inject interrupt.

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

--
			Gleb.
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