IOMMU in bypass mode by default on ARM64, instead of command line option

[Will Deacon]

On Mon, Mar 06, 2017 at 12:34:36PM +0000, Robin Murphy wrote:
> On 04/03/17 16:59, Sunil Kovvuri wrote:
> > I did see some patches submitted earlier to enable user to boot kernel
> > with SMMU in bypass mode using a command line parameter.
> > 
> > Idea sounds good, but just wondering if it would be better if things are done in
> > reverse i.e put SMMU in bypass mode by default on host and provide
> > option to user to enable SMMU via bootargs. Reason i am saying this is
> > with SMMU
> > enabled on host, performance of devices drops down to pathetic levels, not
> > because of HW but mostly because of ARM IOMMU implementation in kernel.

[...]

> > Performance with SMMU enabled drops to <1/5th on NVMe disk and on NIC
> > it's down to nothing. Problem seems to be with high contention for
> > locks which are
> > being used for IOVA maintenance and while updating translation tables inside
> > ARM SMMUv2/v3 drivers.
> 
> Yes, we're well aware that there's a whole load of potential lock
> contention where there doesn't really need to be. The performance
> optimisation effort has mostly been waiting for sufficiently big
> systems/workloads to start appearing in order to measure and justify it.
> Consider yourself the winner :)

Yup. Given that you have numbers (thank you!), then we'll bump the priority
of this work and try to get something out that you can test.

I'll also respin my bypass series ASAP.

Will