Re: [Xen-devel] (v2) Design proposal for RMRR fix

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.01.15 at 10:41, <kevin.tian@xxxxxxxxx> wrote:
>>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Monday, January 12, 2015 5:33 PM
>> >>> On 12.01.15 at 09:46, <kevin.tian@xxxxxxxxx> wrote:
>> >>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> Sent: Friday, January 09, 2015 6:35 PM
>> >> >>> On 09.01.15 at 11:10, <kevin.tian@xxxxxxxxx> wrote:
>> >> >>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> >> The question isn't about migrating with devices assigned, but about
>> >> >> assigning devices after migration (consider a dual vif + SR-IOV NIC
>> >> >> guest setup where the SR-IOV NIC gets hot-removed before
>> >> >> migration and a new one hot-plugged afterwards).
>> >> >>
>> >> >> Furthermore any tying of the guest memory layout to the host's
>> >> >> where the guest first boots is awkward, as post-migration there's
>> >> >> not going to be any reliable correlation between the guest layout
>> >> >> and the new host's.
>> >> >
>> >> > how can you solve this? like above example, a NIC on node-A leaves
>> >> > a reserved region in guest e820. now it's hot-removed and then
>> >> > migrated to node-b. there's no way to update e820 again since it's
>> >> > only boot structure. then user will still see such awkward regions.
>> >> > since it's not avoidable, report-all in the summary mail looks not
>> >> > causing a new problem.
>> >>
>> >> The solution to this are reserved regions specified in the guest config,
>> >> independent of host characteristics.
>> >
>> > I don't think how reserved regions are specified matter here. My point
>> > is that when a region is reserved in e820 at boot time, there's no way
>> > to erase that knowledge in the guest even when devices causing that
>> > reservation are hot removed later.
>> 
>> I don't think anyone ever indicated that such erasure would be
>> needed/wanted - I'm not sure how you ended up there.
>> 
> 
> I ended here to indicate that report-all which gives user more reserved
> regions than necessary is not a weird case since above scenario can also
> create such fact. User shouldn't set expectation about reserved region
> layout. and this argument is necessary to support our proposal of using
> report-all. :-)

The fact that ranges can't be removed from a guest's memory map
is irrelevant - there's simply no question that this is that way. The
main counter argument against report-all remains: It may result in
unnecessarily little low memory in guests not needing all of the host
regions to be reserved for them.

Jan


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