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d="scan'208";a="4578011" From: Paul Durrant To: Anthony Perard Thread-Topic: [PATCH 2/2] xen-bus: Avoid rewriting identical values to xenstore Thread-Index: AQHVWAGpK6THQS4Lrk6EcsKhvd7AqqcFuu6QgAEaRACAACIz8P//7XAAgAAh8kA= Date: Thu, 22 Aug 2019 11:25:44 +0000 Message-ID: <40872481b00f4dbeb82ab540e99121a6@AMSPEX02CL03.citrite.net> 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> <20190822111730.GK1289@perard.uk.xensource.com> In-Reply-To: <20190822111730.GK1289@perard.uk.xensource.com> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-detected-operating-system: by eggs.gnu.org: FreeBSD 9.x X-Received-From: 216.71.145.153 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" > -----Original Message----- > From: Anthony PERARD > Sent: 22 August 2019 12:18 > To: Paul Durrant > Cc: qemu-devel@nongnu.org; Stefano Stabellini ; x= en-devel@lists.xenproject.org > Subject: Re: [PATCH 2/2] xen-bus: Avoid rewriting identical values to xen= store >=20 > 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 ac= tually 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 stat= e is written to xenstore. >=20 > Let's see: > * step 1 (initial states in xenstore and QEMU) > xenstore/frontend/state =3D 4 > xendev->frontend_state =3D 4 > * step 2 (frontend changes state in xenstore) > xenstore/frontend/state =3D 5 > * step 3 (watch event received by QEMU) > xen_device_frontend_changed() > state =3D read(xenstore/frontend/state) (state=3D5) > xen_device_frontend_set_state(state) > xendev->frontend_state !=3D state (4!=3D5) > xendev->frontend_state =3D state > xenstore/frontend/state =3D state > * step 4 > # watch event triggers xen_device_frontend_changed() again but > # this time xendev->frontend_state =3D=3D xenstore/frontend_state >=20 > This is how QEMU writes to xenstore an identical value. >=20 > That behavior might be an issue if the frontend changes the value after > QEMU have read it but before QEMU writes it again. Ah, ok, so the problem is actually limited to frontend state because that i= s written by both frontend and backend, so whether QEMU writes an updated f= rontend state to xenstore needs to be controlled. It's only called in two p= laces xen_device_frontend_changed() and xen_device_realize(). The write to = xenstore should be avoided in the former case, but not the latter. So addin= g a 'publish' boolean and using that to determine whether the write to xens= tore is done seems like the right approach. But I don't think any change is= needed to xen_device_backend_set_online() or xen_device_backend_set_state(= ), is it? Paul >=20 > Also, it's nice to avoid extra work. >=20 > -- > Anthony PERARD