Re: arm-smmu-v3 sharing SID

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Thu, May 18, 2023 at 11:44:56AM +0000, Peng Fan wrote:
> > Subject: Re: arm-smmu-v3 sharing SID
> > 
> > On 2023-05-18 11:46, Peng Fan wrote:
> > > Hi all,
> > >
> > > Current arm-smmu-v3 driver does not support sharing SID, If there are
> > > two devices sharing one SID, the smmu-v3 driver will report "stream x
> > > already in tree".
> > >
> > > We have an SOC that using smmu-v3, only supports limited SIDs. From my
> > > understanding, the smmu-v3 hardware supports SID sharing, but linux
> > > driver not support that.
> > 
> > Ugh, we've always hoped that nobody would build such a thing, since
> > SMMUv3 allows for plenty of stream IDs for any reasonable system (Arm's
> > implementations support at least 24 bits), and aliasing at the StreamID level
> > does rather compromise the usefulness.
> 
> We only support max 64 SIDs. So I am thinking to share SID, since we
> have lots of dma channels.
> 
> > 
> > > I would like know is it ok to add support for sharing SID?
> > > Something like to use common rb_add/find, not
> > > smmu-v3 driver local rb tree add/find.
> > 
> > Not sure what you're thinking there - the StreamID tree is still private to the
> > SMMU instance either way, and the existing arm_smmu_find_master()
> > function is ideal for arm_smmu_device_group() to detect aliasing devices
> > and group them appropriately. Then you also need some way to skip
> > updating STEs that have already been touched for a previous device in the
> > group (basically generalising what arm_smmu_install_ste_for_dev()
> > currently does for duplicate StreamIDs owned by one device).
> 
> Thanks for sharing insights. BTW, would you plan to add the support?
> 
> > 
> > I'm fairly confident in reasoning about this for basic .attach_dev purposes,
> > since I did implement the equivalent for SMMUv2, but I'm not sure I even
> > want to think about what it would mean for SVA, PASIDs and nesting...

We should disable all these things for multi-device groups, it's not worth
the headache. I think we used to but I can't find it anywhere. That said,
I guess only PCIe PRI (not upstream) absolutely won't work with multiple
devices sharing a SID, since page responses are routed back by RID to the
endpoint.

Stall and by extension SVA can't really be supported with multiple devices
per SID either, but that's mainly a software complexity issue (we'd need
to guess which device it comes from, or report the fault for all devices).
PASID and nesting might work transparently due to iommu_group, though I'd
rather not think about that either.

It may boil down to updating the smmu->streams structure to support
multiple masters per stream (maybe simplify the driver first by moving to
a xarray [1]). That would provide a refcount for each SID and allow to
only write the STE once on setup and teardown. Since arm_smmu_find_master()
is only used to handle I/O page faults (stall/PRI which we won't support
in this case), it could return NULL for multi-master streams.

Thanks,
Jean

[1] https://lore.kernel.org/linux-iommu/ecb3725c-27c4-944b-b42c-f4e293521f94@arm.com/