Re: [Xen-devel] [PATCH] AMD IOMMU: Use global interrupt remapping table by default

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

Wei,

Can you be more specific about which BIOSes behave poorly with
per-device intremap tables, and why?

The problem with a global intremap table is that, AFAICT, it's not
fundamentally compatible with per-cpu IDTs.  With per-cpu IDTs,
different devices may end up with interrupts mapped to different cpus
but the same vector (i.e., device A mapped to cpu 9 vector 67, cpu B
mapped to cpu 12 vector 67).  This is by design; the whole point of
the per-cpu IDTs is to avoid restricting the number of IRQs to the
number of vectors.   But it seems that the intremap table only maps
vectors, not destination IDs; so in the example above, both devices'
interrupts would end up being remapped to the same place, causing one
device driver to get both sets of interrupts, and the other to get
none.

Do I understand correctly?  If so, it seems like we should switch to
per-device intremap tables by default; and if we're using a global
intremap table, we need to somehow make sure that vectors are not
shared across cpus.

 -George

On Wed, Oct 28, 2009 at 4:32 PM, Wei Wang2 <wei.wang2@xxxxxxx> wrote:
> Using a global interrupt remapping table shared by all devices has better
> compatibility with certain old BIOSes. Per-device interrupt remapping table
> can still be enabled by using a new parameter "amd-iommu-perdev-intremap".
> Thanks,
> Wei
>
> Signed-off-by: Wei Wang <wei.wang2@xxxxxxx>
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