Re: [Xen-devel] xen/arm: Hiding SMMUs from Dom0 when using ACPI on Xen

[Julien Grall <julien.grall@xxxxxxx>]

On 27/02/17 13:23, Vijay Kilari wrote:
> Hi Julien,
Hello Vijay,

> On Wed, Feb 22, 2017 at 7:40 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
> > Hello,
> >
> > There was few discussions recently about hiding SMMUs from DOM0 when using
> > ACPI. I thought it would be good to have a separate thread for this.
> >
> > When using ACPI, the SMMUs will be described in the IO Remapping Table
> > (IORT). The specification can be found on the ARM website [1].
> >
> > For a brief summary, the IORT can be used to discover the SMMUs present on
> > the platform and find for a given device the ID to configure components such
> > as ITS (DeviceID) and SMMU (StreamID).
> >
> > The appendix A in the specification gives an example how DeviceID and
> > StreamID can be found. For instance, when a PCI device is both protected by
> > an SMMU and MSI-capable the following translation will happen:
> >         RID -> StreamID -> DeviceID
> >
> > Currently, SMMUs are hidden from DOM0 because they are been used by Xen and
> > we don't support stage-1 SMMU. If we pass the IORT as it is, DOM0 will try
> > to initialize SMMU and crash.
> >
> > I first thought about using a Xen specific way (STAO) or extending a flag in
> > IORT. But that is not ideal.
> >
> > So we would have to rewrite the IORT for DOM0. Given that a range of RID can
> > mapped to multiple ranges of DeviceID, we would have to translate RID one by
> > one to find the associated DeviceID. I think this may end up to complex code
> > and have a big IORT table.
> Why can't we replace Output base of IORT of PCI node with SMMU output base?.
> I mean similar to PCI node without SMMU, why can't replace output base
> of PCI node with
> SMMU's output base?.
Because I don't see anything in the spec preventing one RC ID mapping to 
produce multiple SMMU ID mapping. So which output base would you use?
> The issue I see is RID is [15:0] where is DeviceID is [17:0].
>
> > However, given that DeviceID will be used by DOM0 to only configure the ITS.
> > We have no need to use to have the DOM0 DeviceID equal to the host DeviceID.
> > So I think we could simplify our life by generating DeviceID for each RID
> > range.
> If DOM0 DeviceID != host Device ID, then we cannot initialize ITS using DOM0
> ITS commands (MAPD). So, is it concluded that ITS initializes all the devices
> with platform specific Device ID's in Xen?.
Initializing ITS using DOM0 ITS command is a workaround until we get PCI 
passthrough done. It would still be possible to implement that with 
vDeviceID != pDeviceID as Xen would likely have the mapping between the 
2 DeviceID.
Cheers,

--
Julien Grall

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