linux-kernel - Re: [PATCH 00/10] KVM PCIe/MSI passthrough on ARM/ARM64

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Date:	Mon, 1 Feb 2016 14:03:51 +0000
From:	Will Deacon <will.deacon@....com>
To:	Eric Auger <eric.auger@...aro.org>
Cc:	Alex Williamson <alex.williamson@...hat.com>, eric.auger@...com,
	christoffer.dall@...aro.org, marc.zyngier@....com,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
	kvm@...r.kernel.org, Bharat.Bhushan@...escale.com,
	pranav.sawargaonkar@...il.com, p.fedin@...sung.com,
	suravee.suthikulpanit@....com, linux-kernel@...r.kernel.org,
	patches@...aro.org, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 00/10] KVM PCIe/MSI passthrough on ARM/ARM64

On Fri, Jan 29, 2016 at 10:25:52PM +0100, Eric Auger wrote:
> On 01/29/2016 08:33 PM, Alex Williamson wrote:
> >>> We know that x86 handles MSI vectors specially, so there is some
> >>> hardware that helps the situation.  It's not just that x86 has a fixed
> >>> range for MSI, it's how it manages that range when interrupt remapping
> >>> hardware is enabled.  A device table indexed by source-ID references a
> >>> per device table indexed by data from the MSI write itself.  So we get
> >>> much, much finer granularity,
> >> About the granularity, I think ARM GICv3 now provides a similar
> >> capability with GICv3 ITS (interrupt translation service). Along with
> >> the MSI MSG write transaction, the device outputs a DeviceID conveyed on
> >> the bus. This DeviceID (~ your source-ID) enables to index a device
> >> table. The entry in the device table points to a DeviceId interrupt
> >> translation table indexed by the EventID found in the msi msg. So the
> >> entry in the interrupt translation table eventually gives you the
> >> eventual interrupt ID targeted by the MSI MSG.
> >> This translation capability if not available in GICv2M though, ie. the
> >> one I am currently using.
> >>  
> >> Those tables currently are built by the ITS irqchip (irq-gic-v3-its.c)

That's right. GICv3/ITS disambiguates the interrupt source using the
DeviceID, which for PCI is derived from the Requester ID of the endpoint.
GICv2m is less flexible and requires a separate physical frame per guest
to achieve isolation.

Will