Re: [Xen-devel] [PATCH] Revert "xen-hvm: increase maxmem before calling xc_domain_populate_physmap"

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 16 Jun 2015, Stefano Stabellini wrote:
> 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.dunlap@xxxxxxxxxxxxx>
> > > ---
> > > I do think this is the right approach, but I'm mainly sending this is
> > > mainly to open up discussion.
> > > 
> > > CC: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> > > CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> > > CC: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > > CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > 
> > 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?
 
I think it would be more useful to add a check to libxl to explicitly
refuse to create any guests with more than 3 emulated nics.

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