From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52195) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Y0jpn-0000vC-Bh for qemu-devel@nongnu.org; Mon, 15 Dec 2014 23:34:08 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Y0jpl-0007PQ-KA for qemu-devel@nongnu.org; Mon, 15 Dec 2014 23:34:07 -0500 Received: from ozlabs.org ([2401:3900:2:1::2]:54498) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Y0jpl-0007PD-1k for qemu-devel@nongnu.org; Mon, 15 Dec 2014 23:34:05 -0500 Date: Tue, 16 Dec 2014 15:33:54 +1100 From: David Gibson Message-ID: <20141216043354.GK23547@voom.fritz.box> References: <1418361995-24091-1-git-send-email-david@gibson.dropbear.id.au> <20141216041330.GA19675@grmbl.mre> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="J2uG6jHjFLimDtBY" Content-Disposition: inline In-Reply-To: <20141216041330.GA19675@grmbl.mre> Subject: Re: [Qemu-devel] [PATCHv2] Fix virtio-serial migration on bi-endian targets List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Amit Shah Cc: mst@redhat.com, aik@ozlabs.ru, rusty@rustcorp.com.au, qemu-devel@nongnu.org, agraf@suse.de, borntraeger@de.ibm.com, mdroth@us.ibm.com --J2uG6jHjFLimDtBY Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Dec 16, 2014 at 09:43:30AM +0530, Amit Shah wrote: > On (Fri) 12 Dec 2014 [16:26:35], David Gibson wrote: > > On a bi-endian target, with a guest in the non-default endian mode, > > attempting to migrate twice in a row with a virtio-serial device wil > > cause a qemu SEGV on the second outgoing migration. > >=20 > > The problem is that virtio_serial_save_device() (and other places) expe= ct > > VirtIOSerial->config to be in current guest endianness. On a fresh boo= t, > > virtio_serial_device_realize() will initialize VirtIOSerial->config in > > default endianness. It's assumed the guest OS will make its true > > endianness known before the device is reset and initialized, then > > vser_reset adjusts VirtIOSerial->config into the new endianness. > >=20 > > But on an incoming migration, the device isn't reset (after all the gue= st > > has a running driver as far as it's concerned), which means that > > VirtIOSerial->config retains its default endianness value from > > virtio_serial_device_realize(). > >=20 > > On a subsequent outgoing migration, virtio_serial_save_device() attempts > > to interpret VirtIOSerial->config.max_nr_ports in current endianness wh= en > > its actually in default endianness and then runs off the end of the > > ports_map array in the loop immediately afterwards. > >=20 > > We could fix this by adjusting VirtIOSerial->config into the correct > > current endianness after an incoming migration. But a better fix is ju= st > > to get rid of VirtIOSerial->config entirely: > > * The virtio-serial config space is not settable, it always contains t= he > > values set at initialization > > * AFAICT "rows" and "cols" have never actually been used for anything = and > > are always zero. >=20 > There were patches on the list a few years back to add resizing > support. Well, apparently they were never merged, because I can't see a thing in the git history. > Also, ppc and s390 people were using this feature (why else would it > have been implemented?) -- since you're saying they're not in use, I > suppose ppc doesn't use it. CC'ing s390 people for comment. >=20 > > * "max_nr_ports" is initialized from > > VirtIOSerial->serial.max_virtserial_ports (host endian) > >=20 > > So instead of maintaining this pointless guest-endian cache of the conf= ig > > data, we can just construct it directly into the correct current guest > > endian in the get_config hook. Current users of ->config can instead u= se > > the sources from which the config values were derived, which means they > > don't have to mess about with converting from guest endian at all. >=20 > I'd agree with this approach when I have confirmation no one actually > uses the {rows,cols}. I've grepped the tree and searched through git history. I'm pretty sure they're not used. And even if we do want to use them, this approach would still be sound - it would make more sense to hold the rows and cols info in host endian (which is well defined and static, at least) and just byteswap it on get_config() where necessary. > Since qemu doesn't use the rows and cols, it doesn't matter what > settings the dest host has; otherwise the dest host would have had to > adjust the guest to use the dest's settings for rows and cols after a > migration. Also, for "new" guests, they should use the control vq > command to adjust the rows and cols -- and they're not migrated since > they're not guest state. Right, I came to the same conclusion (see my reply to agraf). [snip] > > @@ -552,14 +552,14 @@ static void virtio_serial_save_device(VirtIODevic= e *vdev, QEMUFile *f) > > uint32_t nr_active_ports; > > unsigned int i, max_nr_ports; > > =20 > > - /* The config space */ > > - qemu_put_be16s(f, &s->config.cols); > > - qemu_put_be16s(f, &s->config.rows); > > + max_nr_ports =3D s->serial.max_virtserial_ports; > > =20 > > - qemu_put_be32s(f, &s->config.max_nr_ports); > > + /* Used to be config space, now redundant */ > > + qemu_put_be16(f, 0); > > + qemu_put_be16(f, 0); > > + qemu_put_be32(f, virtio_tswap32(vdev, max_nr_ports)); >=20 > Can you split this patch so the config change and the max_nr_ports > change are separate? The max_nr_ports could similarly be ignored by > dest, right? Um.. I'm not exactly sure where you're drawing the distinction between the two parts. Are you thinking patch 1) make config.max_nr_ports unused, by using max_virtserial_ports instead patch 2) eliminate the config field entirely --=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 --J2uG6jHjFLimDtBY Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQIcBAEBAgAGBQJUj7YyAAoJEGw4ysog2bOS3VgP/1U6aWLfMYeaZPLacqBK8QUU hpGaKKZQAheaBrFdxS3nkZ+B/5vZKbEx2IBnybQqrk61Sg8z22Q5KPkmipt4w5WU 1CLIImrWW4AyUT/odRZu54HZNI8RAApymmi0LOZGSw+++5I6f/0TZT6wdDCrRiRB MjXkOzE46raq56jm9DVUpFvxILFau5oQB/0W8kDdNG4gCfL1E3JkJVINlcUNGNsv NPiQmEEcVc59mwUatImTPkAwFSEu1incxshOj21sHbttlHK1dwbg6WZ7AnmlB7OJ 0ffFYTGvJ3As3MzTFfbFnJA3Cwixw1uCSy50oTZB8FNTBFnvJFX7iFbx6AVx65vc LobK6LmOpAZgEMZ7bgAgSePJ/k7PzkJlKNgBY/GkK0NJmBSX7rF6hBYXuUo7lReh 8ecH0W47m1MtI2SZMS8scdOwmV6HiGnuQlSOe6NvOn8ztU/9bLEMJ/6pmbowHVkL t3a9E2R4yPN7AMRdHLos5PAgKL5NXCyC3X9mKkLKABGZxQ5N6bjsMlZNlPiWIyFk D5BbJQeCoppmZRcHdBGHs1e+MhnJ+P0foJ6BhuRAf2/BvEySU81TKYByvoAVFv9N 9HK1DliTzCW0xkFS8fOCgd/kCFlOt8pN9d5k+O/OiRAsMC5vai9DnZJkDEJeCcpy fUtkm7n8rGlQw1AMKBfR =2HNW -----END PGP SIGNATURE----- --J2uG6jHjFLimDtBY--