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

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Date:	Wed, 26 Aug 2009 13:04:09 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Gleb Natapov <gleb@...hat.com>
cc:	Avi Kivity <avi@...hat.com>, "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, 26 Aug 2009, Gleb Natapov wrote:

> 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 guess that's going to completely throw off-chart your RT performance, 
doesn't it?


- Davide


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