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

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Message-ID: <alpine.DEB.2.00.0908261201570.7780@makko.or.mcafeemobile.com>
Date:	Wed, 26 Aug 2009 12:13:04 -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 08:45 PM, Davide Libenzi wrote:
> > OK, if I get it correctly, there is one eventfd signaler (the device), and
> > one eventfd reader (the hypervisor), right?
> > Each hypervisor listens for multiple devices detecting state changes, and
> > associating the eventfd "line" to the IRQ number by some configuration
> > (ala PCI), right?
> >    
> 
> Yes.  The PCI stuff happens in userspace, all the hypervisor sees is "this
> eventfd is IRQ 10".  There may be multiple eventfds routed to one IRQ
> (corresponding to a shared IRQ line).

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;
}




- Davide


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