From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39388) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Ze3ID-0000Rm-5q for qemu-devel@nongnu.org; Mon, 21 Sep 2015 11:46:14 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Ze3IC-0008Ir-7V for qemu-devel@nongnu.org; Mon, 21 Sep 2015 11:46:13 -0400 References: <1442479781-20164-1-git-send-email-thuth@redhat.com> <20150918110552.6487a506@bahia.local> <20150921021000.GI20331@voom.fritz.box> <55FF9CE2.7060403@redhat.com> From: Eric Blake Message-ID: <56002641.8050905@redhat.com> Date: Mon, 21 Sep 2015 09:46:09 -0600 MIME-Version: 1.0 In-Reply-To: <55FF9CE2.7060403@redhat.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="KwVd9a4kutKgTtefoujasD94omPHw8olQ" Subject: Re: [Qemu-devel] [PATCH v4] ppc/spapr: Implement H_RANDOM hypercall in QEMU List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Thomas Huth , David Gibson , Greg Kurz Cc: agraf@suse.de, kvm-ppc@vger.kernel.org, qemu-devel@nongnu.org, michael@ellerman.id.au, qemu-ppc@nongnu.org, amit.shah@redhat.com, sam.bobroff@au1.ibm.com This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --KwVd9a4kutKgTtefoujasD94omPHw8olQ Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On 09/21/2015 12:00 AM, Thomas Huth wrote: >>> This being said, I am not sure about the use case where a user has a = hwrng >>> capable platform and wants to run guests without any hwrng support at= all is >>> an appropriate default behavior... I guess we will find more users th= at want >>> in-kernel being the default if it is available. >>> >>> The patch below modifies yours to do just this: the pseudo-device is = only >>> created if hwrng is present and not already created. >> >> I have mixed feelings about this. On the one hand, I agree that it >> would be nice to allow H_RANDOM support by default. On the other hand= >> the patch below leaves no way to turn it off for testing purposes. It= >> also adds another place where the guest hardware depends on the host >> configuration, which adds to the already substantial mess of ensuring >> that source and destination hardware configuration matches for >> migration. >=20 > I thought about this question on the weekend and came to the same > conclusion. I think if we want to enable this by default, it likely > should rather be done at the libvirt level instead? >=20 Adding hardware by default to existing machine types has been a bane to libvirt usage in the past. If upgrading from an old qemu to a new one suddenly turns on new guest-visible hardware with no change to the command line, then libvirt has a much harder time migrating that guest. It's okay to have a new machine type turn on a feature by default, and to have knobs so that the feature can be turned on even for older machine types, but experience has shown that any new feature MUST come with knobs and a way to learn if the feature can be turned on/off, rather than just blindly assuming that turning it on is the right thing. --=20 Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org --KwVd9a4kutKgTtefoujasD94omPHw8olQ Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 Comment: Public key at http://people.redhat.com/eblake/eblake.gpg Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/ iQEcBAEBCAAGBQJWACZBAAoJEKeha0olJ0Nqt+YIAJ5/WYno1b/XP9QWcG/nh7R8 KQd43CIigLP+AydIJ2bHKyY8mujKduj1yfXjkeAcCJDocbvu9vnGmBcDRd6KYAbc YO17NkVlc9hmjxC2+R+dNmh7mT0QRN9owxG27cBbAoc7kNpiKk052dIf2JsTbv7V P17lnVBkpcXH7r9toQ4C+pjwx3DgYe8RA5TrHuoKERTjtYxiOaUT+VcNnhAwqE2s P2rXK5Gdm6d7V7rLT6v1hq5VMY9g+EEQizAavqzfdBSIheO9monsyTuBWSu6ZF8G 0+O7izxL3eH8FwAkA/FoUGOi9pSmQoCyHV3fJ4sqn2UT6Cur5WM/xom1zpRdJ9Q= =wt3p -----END PGP SIGNATURE----- --KwVd9a4kutKgTtefoujasD94omPHw8olQ--