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

[George Dunlap <george.dunlap@eu.citrix.com>]

On 06/10/2015 01:55 PM, 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.)

Just to make it clear what the situation is (to the best of my knowledge):

QEMU 2.2 and before:
 * A VM assigned more than 3 NICs would fail during qemu start-up
 * A VM assigned 3 or fewer NICs can be created and migrated successfully.

QEMU 2.3 (most recent release):
 * A VM assigned more than 3 NICs can be created successfully, but not
migrated afterwards
 * A VM assigned 3 or fewer NICs can be both created and migrated.
(Stefano has done a few tests to verify this and it seems to be accurate.)

It's unlikely that the "proper fix" descibed in this mail will be ready
for 2.4, so if this patch is accepted, 2.4 will look like 2.2.

 -George