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

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Message-ID: <20160203131057.GA20217@arm.com>
Date:	Wed, 3 Feb 2016 13:10:58 +0000
From:	Will Deacon <will.deacon@....com>
To:	Christoffer Dall <christoffer.dall@...aro.org>
Cc:	Eric Auger <eric.auger@...aro.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	eric.auger@...com, 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 Wed, Feb 03, 2016 at 01:50:47PM +0100, Christoffer Dall wrote:
> On Mon, Feb 01, 2016 at 02:03:51PM +0000, Will Deacon wrote:
> > 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.
> > 
> We should still support MSI passthrough with a single MSI frame host
> system though, right?

I think we should treat the frame as an exclusive resource and assign it
to a single VM.

> (Users should just be aware that guests are not fully protected against
> misbehaving hardware in that case).

Is it confined to misbehaving hardware? What if a malicious/buggy guest
configures its device to DMA all over the doorbell?

Will