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

[Christoffer Dall]

On Wed, Feb 03, 2016 at 01:10:58PM +0000, Will Deacon wrote:
> 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.

so on a single frame GICv2m system, either your host or a single VM gets
to do MSIs...

> 
> > (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?
> 
I guess not, I suppose we can't trap any configuration access and
mediate that for any device.  Bummer.

-Christoffer