From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37307) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dsLG1-00014k-Rp for qemu-devel@nongnu.org; Wed, 13 Sep 2017 23:56:07 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dsLFz-0000xp-9P for qemu-devel@nongnu.org; Wed, 13 Sep 2017 23:56:05 -0400 Date: Thu, 14 Sep 2017 13:48:00 +1000 From: David Gibson Message-ID: <20170914034800.GJ3972@umbus.fritz.box> References: <20170911095059.101e0cfc@bahia.lan> <20170911093032.GA2857@work-vm> <20170911104854.GB2784@umbus.fritz.box> <20170912162100.GD2225@work-vm> <1bb6a27d-89a5-2e70-6976-74ab24430ec4@ilande.co.uk> <4f0cb74a-ae00-5921-291c-49c9acdb3e02@ilande.co.uk> <20170913060203.GG7550@umbus.fritz.box> <760c3889-8561-9bce-e565-ccbcb9eb8cf5@ilande.co.uk> <20170913191309.56bf8fa4@bahia.lan> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="LQAwcd5tHl0Qlnzi" Content-Disposition: inline In-Reply-To: <20170913191309.56bf8fa4@bahia.lan> Subject: Re: [Qemu-devel] [Qemu-ppc] [PATCH 2/4] ppc: add CPU IRQ state to PPC VMStateDescription List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Greg Kurz Cc: Mark Cave-Ayland , Alexey Kardashevskiy , lvivier@redhat.com, qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , qemu-ppc@nongnu.org --LQAwcd5tHl0Qlnzi Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Sep 13, 2017 at 07:13:09PM +0200, Greg Kurz wrote: > On Wed, 13 Sep 2017 17:44:54 +0100 > Mark Cave-Ayland wrote: >=20 > > On 13/09/17 07:02, David Gibson wrote: > >=20 > > >>> Alexey - do you recall from your analysis why these fields were no > > >>> longer deemed necessary, and how your TCG tests were configured? = =20 > > >> > > >> I most certainly did not do analysis (my bad. sorry) - I took the pa= tch > > >> from David as he left the team, fixed to compile and pushed away. I = am also > > >> very suspicions we did not try migrating TCG or anything but pseries= =2E My > > >> guest that things did not break (if they did not which I am not sure= about, > > >> for the TCG case) because the interrupt controller (XICS) or the > > >> pseries-guest took care of resending an interrupt which does not see= m to be > > >> the case for mac99. =20 > > >=20 > > > Right, that's probably true. The main point, though, is that these > > > fields were dropped a *long* time ago, when migration was barely > > > working to begin with. In particular I'm pretty sure most of the > > > non-pseries platforms were already pretty broken for migration > > > (amongst other things). > > >=20 > > > Polishing the mac platforms up to working again, including migration, > > > is a reasonable goal. But it can't be at the expense of pseries, > > > which is already working, used in production, and much better tested > > > than mac99 or g3beige ever were. =20 > >=20 > > Oh I completely agree since I'm well aware pseries likely has more users > > than the Mac machines - my question was directed more about why we > > support backwards migration. > >=20 >=20 > Downstream support backward migration because end users/customers ask for= it > for maximum flexibility when it comes to move workloads around different = systems > with different QEMU versions. This is fairly usual in data centers with m= any > systems. One specific case where this matters is if you want to update the qemu version on an entire cluster of hosts. Management layers like oVirt/RHV allow you to do that without disrupting guests by migrating them off each host, one by one, allowing you to remove it from the cluster, update and then reinsert after which guests may be migrated onto it again. But that process could obviously take a fair while for a big cluster; days or even weeks. In the meantime you have a cluster with mixed qemu versions, and since oVirt/RHV/whatever will also migrate guests around the cluster to load balance, that means the possibility of a backwards migration. > As others already said, breaking things upstream may turn downstream work > into a nightmare (and FWIW, most of the people working on ppc are also > involved in downstream work). Right. And I even used to work as a RHV support tech too :). --=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 --LQAwcd5tHl0Qlnzi Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEdfRlhq5hpmzETofcbDjKyiDZs5IFAlm5++0ACgkQbDjKyiDZ s5It+Q/8CXcF8Fejbg7qs9GOVWfXNLUfYziLyPLGUFNZZv2tvCxcRmhduxMJmfKj uwWeoHs4dVs7yetekRXBgLktgG5adLNFiThCC4fd44smACXRH2ie3PtzshzlrqvX Tsznjg5Rfb5SpBmRceTLTPPav3x0B1wpMei3isdefcmhMvSn5GnXq3+hVTBneJUK polvrY2pDgqKUhlYZu6tl0O0bs3OIfoRf8+qEQODWaYL3ZRZuLyfwf3KtQGhtB+J 0psGic06gy5XfsGOGP4USuwDrltUD3XAl1nyL3rSAZlnWGzyevK+6Da6z2iDpoOV vHpx8IVhbR5XcmumS7nx7Aib8a8kHpwf9QWj8aDuS+4HeRaGHWgkDQn2U1NyW398 2QjYjsTw8Fh604RmA7YigdvRl7VNYTZzCDUZL1UBH3HRLS/qnPlwKkdTe3Nohzsr CaPKiOmpn8X/DT/JZb1cdc30vC3+H8EyTtLJkNKUJVFqhtlpqsxbb4OLBthmqiK1 A/sQ50cJgrWg2Iz8vyO3P9EYsiwE2X0qlFSVRcNB+ujck31Xzu7Esf9gi5VCpBRF RybUGRZkxrYcRL3aK/p3buv8dWGW4Zxbtm9liSvP5OifftRaz+uNjKt/a4YKoElS EcyK4j3P69lyKW9gqGY2U0v0IHXhJyTZwEVHYUafD3QuvdjK/nA= =48j2 -----END PGP SIGNATURE----- --LQAwcd5tHl0Qlnzi--