From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932375AbZHCQ6V (ORCPT ); Mon, 3 Aug 2009 12:58:21 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932368AbZHCQ6S (ORCPT ); Mon, 3 Aug 2009 12:58:18 -0400 Received: from mx2.redhat.com ([66.187.237.31]:58165 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932211AbZHCQ6Q (ORCPT ); Mon, 3 Aug 2009 12:58:16 -0400 Date: Mon, 3 Aug 2009 19:57:08 +0300 From: "Michael S. Tsirkin" To: Avi Kivity Cc: davidel@xmailserver.org, gleb@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH-RFC 2/2] eventfd: EFD_STATE flag Message-ID: <20090803165708.GB3630@redhat.com> References: <20090728175538.GC21549@redhat.com> <4A76FDB2.7080706@redhat.com> <20090803151426.GA3630@redhat.com> <4A770260.5000507@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4A770260.5000507@redhat.com> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Aug 03, 2009 at 06:29:36PM +0300, Avi Kivity wrote: > 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(); > } What if write comes before read? > -- > error compiling committee.c: too many arguments to function