On Tue, 16 Jun 2015, Wei Liu wrote: > On Wed, Jun 10, 2015 at 01:55:13PM +0100, George Dunlap wrote: > > This reverts commit c1d322e6048796296555dd36fdd102d7fa2f50bf. > > > > The original commit fixes a bug when assigning a large number of > > devices which require option roms to a guest. (One known > > configuration that needs extra memory is having more than 4 emulated > > NICs assigned. Three or fewer NICs seems to work without this > > functionality.) > > > > However, by unilaterally increasing maxmem, it introduces two > > problems. > > > > First, now libxl's calculation of the required maxmem during migration > > is broken -- any guest which exercised this functionality will fail on > > migration. (Guests which have the default number of devices are not > > affected.) > > > > Secondly, it makes it impossible for a higher-level toolstack or > > administer to predict how much memory a VM will actually use, making > > it much more difficult to effectively use all of the memory on a > > machine. > > > > The right solution to the original problem is to figure out a way for > > qemu to take pages from the existing pool of guest memory, rather than > > allocating more pages. > > > > That fix will take more time to develop than we have until the feature > > freeze. In the mean time, the simplest way to fix the migration issue > > is to revert this change. That will re-introduce the original bug, > > but it's an unusual corner case; and without migration it isn't fully > > functional yet anyway. > > > > Signed-off-by: George Dunlap <george.dun...@eu.citrix.com> > > --- > > I do think this is the right approach, but I'm mainly sending this is > > mainly to open up discussion. > > > > CC: Stefano Stabellini <stefano.stabell...@citrix.com> > > CC: Wei Liu <wei.l...@citrix.com> > > CC: Ian Campbell <ian.campb...@citrix.com> > > CC: Andrew Cooper <andrew.coop...@citrix.com> > > Stefano, Andrew, any comments? > > If we're to do this we need to do it now. > > I think reverting this change in QEMU and relevant changes in libxl > would be the most viable solution to solve this for this release.
Reverting this patch doesn't really solve the problem: instead of breaking on migration when the VM has more than 3 emulated NICs, the VM simply refuses to start in that case. I guess it can be considered a small improvement but certainly not a fix. Given that the migration issue only happens in an "unusual corner case", are we really in a hurry to revert this commit and go back to the failure to start, even before we actually figure out what the proper fix is? _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
