Subject: [PATCH 1/2] irqchip/gicv3-its: Support share device ID

From: [email-masked] (Will Deacon)
Date: Mon, 27 Apr 2015 18:04:23 +0100

On Mon, Apr 27, 2015 at 02:08:10PM +0100, Varun Sethi wrote:
> > >>> In the SMMU/GIC-500-ITS world the iommu isolation ID (the stream ID)
> > >>> and the GIC-ITS device ID are in fact the same ID.
> > >>
> > >> The DeviceID is the "MSI group" you mention. This is what provides
> > >> isolation at the ITS level.
> > >>
> > > [varun] True, in case of a transparent host bridge device Id won't
> > > provide the necessary isolation.
> > 
> > Well, it depends how you look at it. How necessary is this isolation, since
> > we've already established that you couldn't distinguish between these
> > devices at the IOMMU level?
> > 
> [varun] Yes, the devices would fall in the same IOMMU group. So, devices
> would end up sharing the interrupt?

Well, I think that's the crux of the issue here. If IOMMU groups are also
needed to relay constraints to the IRQ subsystem, then perhaps we need a
more general notion of device grouping and ID transformations between
the different levels of group hierarchy.

Will