Re: [Xen-devel] [PATCH 4/4] AMD IOMMU: untie remap and vector maps

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.03.13 at 13:37, George Dunlap <George.Dunlap@xxxxxxxxxxxxx> wrote:
> On Tue, Mar 26, 2013 at 9:05 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>> With the specific IRTEs used for an interrupt no longer depending on
>> the vector, there's no need to tie the remap sharing model to the
>> vector sharing one.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Unfortunately I'm having a bit of trouble "paging in" the technical
> details of why this change was necessary.  I thought that the
> limitation had to do with the remapping table itself.  Are you saying
> that if interrupt X maps to pcpu 0 vector 5, and interrupt Y maps to
> pcpu 2 vector 5, that even if X and Y are in the same table, they will
> still go to the right place?  Could you give me a brief explanation of
> why that is?  (I don't know what IRTE is, and Google is not helpful.)

Previously the remapping table was indexed by vector (combined
with delivery mode, for whatever reason). That way, a particular
vector, no matter which CPU it would have been meant to go to
caused the same IRTE (Interrupt Remapping Table Entry) to be
selected, and with it the same destination ID.

By not using the vector for indexing anymore, a vector allocated
in CPU A's space connects with IRTE x with destination ID set to
(or including, when in cluster or flat mode) whereas CPU B's
identical vector connects to a different IRTE (even when using a
single global remapping table), properly set to the destination ID
for (or including) that CPU.

Jan


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