From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53288) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Y0juv-0002La-CY for qemu-devel@nongnu.org; Mon, 15 Dec 2014 23:39:31 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Y0jup-0000SG-6I for qemu-devel@nongnu.org; Mon, 15 Dec 2014 23:39:25 -0500 Received: from mx1.redhat.com ([209.132.183.28]:45291) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Y0juo-0000Rv-VB for qemu-devel@nongnu.org; Mon, 15 Dec 2014 23:39:19 -0500 Date: Tue, 16 Dec 2014 10:08:49 +0530 From: Amit Shah Message-ID: <20141216043849.GB19675@grmbl.mre> References: <1418361995-24091-1-git-send-email-david@gibson.dropbear.id.au> <20141216041330.GA19675@grmbl.mre> <20141216043354.GK23547@voom.fritz.box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141216043354.GK23547@voom.fritz.box> 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: David Gibson 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 On (Tue) 16 Dec 2014 [15:33:54], David Gibson wrote: > 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. > > > > > > The problem is that virtio_serial_save_device() (and other places) expect > > > VirtIOSerial->config to be in current guest endianness. On a fresh boot, > > > 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. > > > > > > But on an incoming migration, the device isn't reset (after all the guest > > > 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(). > > > > > > On a subsequent outgoing migration, virtio_serial_save_device() attempts > > > to interpret VirtIOSerial->config.max_nr_ports in current endianness when > > > its actually in default endianness and then runs off the end of the > > > ports_map array in the loop immediately afterwards. > > > > > > We could fix this by adjusting VirtIOSerial->config into the correct > > > current endianness after an incoming migration. But a better fix is just > > > to get rid of VirtIOSerial->config entirely: > > > * The virtio-serial config space is not settable, it always contains the > > > values set at initialization > > > * AFAICT "rows" and "cols" have never actually been used for anything and > > > are always zero. > > > > 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. Yes, the submitter didn't follow up after review. > > 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. > > > > > * "max_nr_ports" is initialized from > > > VirtIOSerial->serial.max_virtserial_ports (host endian) > > > > > > So instead of maintaining this pointless guest-endian cache of the config > > > data, we can just construct it directly into the correct current guest > > > endian in the get_config hook. Current users of ->config can instead use > > > 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. > > > > 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(VirtIODevice *vdev, QEMUFile *f) > > > uint32_t nr_active_ports; > > > unsigned int i, max_nr_ports; > > > > > > - /* The config space */ > > > - qemu_put_be16s(f, &s->config.cols); > > > - qemu_put_be16s(f, &s->config.rows); > > > + max_nr_ports = s->serial.max_virtserial_ports; > > > > > > - 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)); > > > > 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 For this patch, I'm just suggesting to only touch cols and rows, and lines that touch the max_nr_ports can be put in 2/2. Eliminating config entirely may not be desirable, but if you want to do that, and mark all fields as 'unused' in the savevm/loadvm functions, go for it :-) Amit