From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50089) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bMSEE-000625-2H for qemu-devel@nongnu.org; Sun, 10 Jul 2016 23:49:55 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bMSE9-0008Td-Um for qemu-devel@nongnu.org; Sun, 10 Jul 2016 23:49:53 -0400 Date: Mon, 11 Jul 2016 13:26:01 +1000 From: David Gibson Message-ID: <20160711032601.GL16355@voom.fritz.box> References: <1467903025-13383-1-git-send-email-bharata@linux.vnet.ibm.com> <1467903025-13383-4-git-send-email-bharata@linux.vnet.ibm.com> <20160708052413.GM14675@voom.fritz.box> <20160708064112.GK25522@in.ibm.com> <20160708073952.GQ14675@voom.fritz.box> <20160708125959.2a5c35bb@nial.brq.redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="nrM5Z5VIJgwP9LWp" Content-Disposition: inline In-Reply-To: <20160708125959.2a5c35bb@nial.brq.redhat.com> Subject: Re: [Qemu-devel] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id for threads of CPU cores List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Igor Mammedov Cc: Bharata B Rao , qemu-devel@nongnu.org, qemu-ppc@nongnu.org, groug@kaod.org, nikunj@linux.vnet.ibm.com, pbonzini@redhat.com --nrM5Z5VIJgwP9LWp Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Jul 08, 2016 at 12:59:59PM +0200, Igor Mammedov wrote: > On Fri, 8 Jul 2016 17:39:52 +1000 > David Gibson wrote: >=20 > > On Fri, Jul 08, 2016 at 12:11:12PM +0530, Bharata B Rao wrote: > > > On Fri, Jul 08, 2016 at 03:24:13PM +1000, David Gibson wrote: =20 > > > > On Thu, Jul 07, 2016 at 08:20:23PM +0530, Bharata B Rao wrote: =20 > > > > > Conditonally set stable_cpu_id for CPU threads that are created a= s part > > > > > of spapr CPU cores. The use of stable_cpu_id is enabled for pseri= es-2.7 > > > > > onwards. > > > > >=20 > > > > > Signed-off-by: Bharata B Rao > > > > > --- > > > > > hw/ppc/spapr_cpu_core.c | 7 +++++++ > > > > > 1 file changed, 7 insertions(+) > > > > >=20 > > > > > diff --git a/hw/ppc/spapr_cpu_core.c b/hw/ppc/spapr_cpu_core.c > > > > > index b104778..0ec3513 100644 > > > > > --- a/hw/ppc/spapr_cpu_core.c > > > > > +++ b/hw/ppc/spapr_cpu_core.c > > > > > @@ -293,8 +293,15 @@ static void spapr_cpu_core_realize(DeviceSta= te *dev, Error **errp) > > > > > for (i =3D 0; i < cc->nr_threads; i++) { > > > > > char id[32]; > > > > > obj =3D sc->threads + i * size; > > > > > + CPUState *cs; > > > > > =20 > > > > > object_initialize(obj, size, typename); > > > > > + cs =3D CPU(obj); > > > > > + > > > > > + /* Use core_id (which is actually cpu_dt_id) as stable C= PU id */ > > > > > + if (cs->has_stable_cpu_id) { > > > > > + cs->stable_cpu_id =3D cc->core_id + i; > > > > > + } =20 > > > >=20 > > > > Testing cs->has_stable_cpu_id here in machine type specific code se= ems > > > > really weird. It's the machine type that knows whether it has a > > > > stable ID to give to the CPU or not, rather than the other way arou= nd. > > > >=20 > > > > Since we haven't yet had a release with cpu cores, I think the right > > > > thing is for cpu_core to unconditionally set the stable ID (and set > > > > has_stable_id to true). =20 > > >=20 > > > Right, we can set cpu_stable_id unconditionally here since this code = path > > > (core realize) will be taken only for pseries-2.7 onwards. has_stable= _id > > > will get set as part of the property we defined in SPAPR_COMPAT_2_7. = =20 > >=20 > > Hrm, that's true. But when you describe it like that it sounds like a > > really non-obvious and fragile dependency between different components. > that's how compat stuff is typically done for devices, > CPUs shouldn't be an exception.=20 > (consistency with other devices helps here in long run) > =20 > > > > The backup path that does thread-based cpu > > > > init, can set has_stable_id to false (if that's not the default). = =20 > > >=20 > > > Default is off, but turning it on for 2.7 will be inherited by 2.6 > > > and others below. Hence I have code to explicitly disable this prop > > > for 2.6 and below via SPAPR_COMPAT_2_6. =20 > >=20 > > This is all seeming terribly awkward. > Typically default is set the way so new machine type doesn't have > to enable it explicitly. >=20 > However the way it's done here helps not to touch/check every user > that uses cpu_index, limiting series impact only to code that > asks for it, it look a lot safer to got this rout for now. >=20 >=20 > > Can we try investigating a > > different approach: > >=20 > > 1. Rename cpu_index to cpu_id, but it's still used in the same > > places it's used. > >=20 > > 2. Remove assumptions that cpu_id values are contiguous or > > dense > > =20 > > 3. Machine type decides whether it wants to populate the cpu_id > > values explicitly, or leave it to generic code to calculate > > them as cpu_index is calculated now. > >=20 > > 4. Ideally, generic code enforces that the machine type populates > > either all or none of the cpu_id values. > >=20 > > Does that seem workable? > Ideally we will get there some day (and may be get rid of cpu_index altog= ether), > but for now it seems too invasive with a lot of chances to introduce non = obvious > regression. Yes, that's a risk. But I'm basically no longer convinced that it's any higher than the risk of the same thing with the current approach. > So I'd keep approach used in this series. --=20 David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson --nrM5Z5VIJgwP9LWp Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQIcBAEBAgAGBQJXgxHJAAoJEGw4ysog2bOSRCAP/R8fEE3lsLiIPwI6NpIIWHbh zZZIhJBNFGskdJo3cgKl/ICjwmHikA0YKQ+AKbrODdUD0MrlGVVoPtyiwuMEs2pQ DuEHGNbGsowx9J6/vuummCWJFK17IeHnUe1CQYw8hcNQWvTEDnFZ8fLramaIJbwF 13ABdYBqzy2uqBp1QRdgAM4aFInsfTAiIZUum3lM5CGkX+U9qGTbQGzxl1qA39nQ Trl+3K78UoMyoWFgrzJAZS2LhBz3potShlrQULtDUroMqvEg70N4uHbrlHF0fCvp pPGTlXT9pY64nLvYQKHQUsTbiS0GPlWKrzePLRf2LUcjhjzA4jxH1/eHYN7lpYh1 FbIB85scY2+/XRLczmJnny5z+956/OmreQRObhKyo6w0zrlHpJvPTIljUp61tqad 7nKybZ/jCnNaUYtanDzYRRuDOBB74Hi0jDlyObuLoAyHV3lVL64hHNqPNvIBrR+m NKpcPg+1f7WZeyU2y6mLl71vQBrQfAnD/czH4ANVghWPSBDEDGVXqB15BCpVZhus /F9j9H/VWb99Ca+dI0KMkYPg8xXsk8DY6BfqTvUWxT+uqoYBovDwoI74nwT+jzze Ac/mbo2+mZBoPGYuivZ3elSaFFVXig/cSb3XwSOvpfX+dxpxCkq+HYHp4+2PDq1x jKSPpzKdQMwTSqoRoiA8 =EvR2 -----END PGP SIGNATURE----- --nrM5Z5VIJgwP9LWp--