Re: ARM SMMU - Stall mode

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

On Thu, Mar 19, 2015 at 06:52:07AM +0000, Edgar E. Iglesias wrote:
> Hi,

Hello Edgar,

> I'm looking at adding models for the SMMU and it's stall feature to
> Xilinx ZynqMP QEMU models. The stall mode is quite problematic to
> model though. So I'm trying to figure out if I can get away without it :-)

Sure -- just make the context bank SCTLR.CFCFG fields RES0 and you'll be
fine. The stall mode is optional in the architecture and not used by
Linux.

> I was trying to figure from the kernel sources if the SMMU Stall mode
> (for stalling and later resuming failed memory accesses) is used by
> the kernel.
> 
> AFAICT, the stall mode is enabled by the SMMU driver but the users of
> the report_iommu_fault() callbacks seem to all just do error reporting.
> Do I understand correctly in that Linux/KVM is currently not really
> depending on the stall function?
> 
> Are there any plans or ongoing work to implement paging features that
> would require the stall/resume feature for IOMMUs?
> E.g swapping of DMA:able memory, CoW etc.
> 
> I noticed that AMD has something in their HSA/KFD framework around it
> but it seems quite AMD specific, please correct me if I'm wrong.

The main issue with the stall model is that it doesn't work nicely with
PCI and is prone to deadlock on systems that haven't been designed with
stalled transactions in mind. If/when Linux gets support for paged DMA
memory this will probably be using ATS/PRI.

Will