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

From: Will Deacon <will.deacon@arm.com>
To: Varun Sethi <Varun.Sethi@freescale.com>
Cc: Marc Zyngier <Marc.Zyngier@arm.com>,
	Stuart Yoder <stuart.yoder@freescale.com>,
	"Minghuan.Lian@freescale.com" <Minghuan.Lian@freescale.com>,
	"linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	"Mingkai.Hu@freescale.com" <Mingkai.Hu@freescale.com>,
	Roy Zang <tie-fei.zang@freescale.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Scott Wood <scottwood@freescale.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 1/2] irqchip/gicv3-its: Support share device ID
Date: Mon, 27 Apr 2015 18:04:23 +0100	[thread overview]
Message-ID: <20150427170423.GC3310@arm.com> (raw)
In-Reply-To: <BN1PR0301MB0627CD95FC31B221DAA644EDEAE90@BN1PR0301MB0627.namprd03.prod.outlook.com>

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