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

To: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] AMD IOMMU: Use global interrupt remapping table by default
From: Wei Wang2 <wei.wang2@xxxxxxx>
Date: Wed, 20 Jul 2011 11:52:41 +0200
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On Tuesday 19 July 2011 16:14:31 George Dunlap wrote:
> Wei,
> Can you be more specific about which BIOSes behave poorly with
> per-device intremap tables, and why?
We found that, in some case, SATA device uses different device ids for dma 
remapping and interrupt remapping. Some early BIOSes cannot handle this 
situation correctly, so if SATA uses device id for DMA to lookup device table 
entry for intremap table and if intremap table is per-device configured, SATA 
device won't get the right table.

> 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.

Yes, obviously a problem...Using shared intremap table, devices uses the same 
vector and delivery mode will end up to the same remapping entry. Is per-cpu 
IDTs enable by default in Xen?

> 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.
I agree to use per-device table by default, since BIOS issue has been fixed 
and per-device table also has some security advantages.


>  -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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