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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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.

Wei.

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