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

[will.deacon@arm.com (Will Deacon)]

Hi Stuart,

First of, thanks for taking the time to explain this in more detail.
Comments inline.

On Fri, Apr 17, 2015 at 03:19:08PM +0100, Stuart Yoder wrote:
> > On Wed, Apr 15, 2015 at 02:18:13PM +0100, Varun Sethi wrote:
> > > Yes, deviceid=stream id (i.e. ICID + other bits). I am not sure if TBU ID
> > > would also be forwarded as a part of stream id to GIC. My understanding is
> > > that TBU ID is forwarded (as a part of the stream ID) to the TCU in case
> > > of a TBU translation miss. In case of the LS2085 PCIe controller you would
> > > have to setup the PCIe device ID to stream ID translation table. We may
> > > have to restrict the number of entries based on the available number of
> > > contexts.
> > 
> > Unfortunately, I'm having a really hard time parsing this thread (some parts
> > of it simply don't make sense; others use non-architectural terms and
> > overall I don't get a feeling for the problem).
> > 
> > Please could you explain your system design step by step so that I can
> > understand (a) what you've built and (b) why the current design of Linux is
> > causing you problems?
> > 
> > Sorry if I'm just being thick, but it's important that we get this right.
> 
> I'll try to summarize some key points about the system...
> 
> System is using a single SMMU-500 (1 TCU, 6 TBUs) and GICv3-ITS.  There are
> PCI, fsl-mc, and platform devices that do DMA.  Devices on the PCI and
> fsl-mc bus generate message interrupts.

Ah cool, so you have multiple buses sharing a single SMMU? That's going to
necessitate some ID remapping in the device-tree. Perhaps you could comment
on Mark Rutland's proposal if it does/doesn't work for you:

  http://lists.infradead.org/pipermail/linux-arm-kernel/2015-March/333199.html


> The flow a message interrupt would take is this:
> 
>     --------------
>       PCI device
>     --------------
>           |
>           | pcidevid + MSI msg
>           |
>           V
>     --------------
>     PCI controller
>       pcidevid ->
>       streamID
>       mapping
>     --------------
>           |
>           | streamID + MSI msg
>           |
>           V
>     --------------
>         SMMU
>     --------------
>           |
>           | streamID + MSI msg
>           |
>           V
>     --------------
>        CCN-504 (Dickens)
>     --------------
>           |
>           | streamID + MSI msg
>           |
>           V

The streamID here as the same as the one coming out of the SMMU, right?
(just trying to out why you have the CCN-504 in the picture).

>     --------------
>       GICv3 ITS    streamID == ITS deviceID
>     --------------
> 
> So, the way things work (at least initially) is that each PCI device maps
> to a single streamID, and thus each device has a separate ITT in 
> the ITS.  So, things should be cool.
> 
> However, there is an improvement we envision as possible due to 
> the limited number of SMMU contexts (i.e. 64).  If there are
> 64 SMMU context registers it means that there is a max of
> 64 software contexts where things can be isolated.  But, say I have
> an SRIOV card with 64 VFs, and I want to assign 8 of the VFs
> to a KVM VM.  Those 8 PCI devices could share the same
> streamID/ITS-device-ID since they all share the same isolation
> context.
> 
> What would be nice is at the time the 8 VFS are being added
> to the IOMMU domain is for the pcidevid -> streamID mapping
> table to be updated dynamically.  It simply lets us make
> more efficient use of the limited streamIDs we have.
> 
> I think it is this improvement that Minghuan had in mind
> in this patch.

Ok, but in this case it should be possible to use a single context bank for
all of the VF streamIDs by configuring the appropriate SMR, no? Wouldn't
that sort of thing be preferable to dynamic StreamID assignment? It would
certainly make life easier for the MSIs.

Will