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d="scan'208";a="4591279" Date: Thu, 22 Aug 2019 12:17:30 +0100 From: Anthony PERARD To: Paul Durrant Message-ID: <20190822111730.GK1289@perard.uk.xensource.com> References: <20190821092020.17952-1-anthony.perard@citrix.com> <20190821092020.17952-3-anthony.perard@citrix.com> <703d5a46d4c74eb4afd93d76b7341efc@AMSPEX02CL03.citrite.net> <20190822102132.GJ1289@perard.uk.xensource.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.12.1 (2019-06-15) X-detected-operating-system: by eggs.gnu.org: FreeBSD 9.x X-Received-From: 216.71.145.155 Subject: Re: [Qemu-devel] [PATCH 2/2] xen-bus: Avoid rewriting identical values to xenstore X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "xen-devel@lists.xenproject.org" , Stefano Stabellini , "qemu-devel@nongnu.org" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Aug 22, 2019 at 11:36:32AM +0100, Paul Durrant wrote: > But, now I look at the code again without your patch applied I don't actually see the problem it is trying to fix. The functions xen_device_[back|front]end_set_state return early if the state being set matches the existing state and hence never get to the line where the state is written to xenstore. Let's see: * step 1 (initial states in xenstore and QEMU) xenstore/frontend/state = 4 xendev->frontend_state = 4 * step 2 (frontend changes state in xenstore) xenstore/frontend/state = 5 * step 3 (watch event received by QEMU) xen_device_frontend_changed() state = read(xenstore/frontend/state) (state=5) xen_device_frontend_set_state(state) xendev->frontend_state != state (4!=5) xendev->frontend_state = state xenstore/frontend/state = state * step 4 # watch event triggers xen_device_frontend_changed() again but # this time xendev->frontend_state == xenstore/frontend_state This is how QEMU writes to xenstore an identical value. That behavior might be an issue if the frontend changes the value after QEMU have read it but before QEMU writes it again. Also, it's nice to avoid extra work. -- Anthony PERARD