Re: [PATCH v12 5/6] iommu/dma: Allow a single FQ in addition to per-CPU FQs

[Niklas Schnelle]

On Fri, 2023-08-25 at 12:11 +0200, Niklas Schnelle wrote:
> In some virtualized environments, including s390 paged memory guests,
> IOTLB flushes are used to update IOMMU shadow tables. Due to this, they
> are much more expensive than in typical bare metal environments or
> non-paged s390 guests. In addition they may parallelize poorly in
> virtualized environments. This changes the trade off for flushing IOVAs
> such that minimizing the number of IOTLB flushes trumps any benefit of
> cheaper queuing operations or increased paralellism.
> 
> In this scenario per-CPU flush queues pose several problems. Firstly
> per-CPU memory is often quite limited prohibiting larger queues.
> Secondly collecting IOVAs per-CPU but flushing via a global timeout
> reduces the number of IOVAs flushed for each timeout especially on s390
> where PCI interrupts may not be bound to a specific CPU.
> 
> Let's introduce a single flush queue mode that reuses the same queue
> logic but only allocates a single global queue. This mode is selected by
> dma-iommu if a newly introduced .shadow_on_flush flag is set in struct
> dev_iommu. As a first user the s390 IOMMU driver sets this flag during
> probe_device. With the unchanged small FQ size and timeouts this setting
> is worse than per-CPU queues but a follow up patch will make the FQ size
> and timeout variable. Together this allows the common IOVA flushing code
> to more closely resemble the global flush behavior used on s390's
> previous internal DMA API implementation.
> 
> Link: https://lore.kernel.org/all/9a466109-01c5-96b0-bf03-304123f435ee@xxxxxxx/
> Acked-by: Robin Murphy <robin.murphy@xxxxxxx>
> Reviewed-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx> #s390
> Signed-off-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
> ---
>  drivers/iommu/dma-iommu.c  | 168 ++++++++++++++++++++++++++++++++++-----------
>  drivers/iommu/s390-iommu.c |   3 +
>  include/linux/iommu.h      |   2 +
>  3 files changed, 134 insertions(+), 39 deletions(-)
> 
> 
---8<---
>  
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 182cc4c71e62..c3687e066ed7 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -409,6 +409,7 @@ struct iommu_fault_param {
>   * @priv:	 IOMMU Driver private data
>   * @max_pasids:  number of PASIDs this device can consume
>   * @attach_deferred: the dma domain attachment is deferred
> + * @shadow_on_flush: IOTLB flushes are used to sync shadow tables
>   *
>   * TODO: migrate other per device data pointers under iommu_dev_data, e.g.
>   *	struct iommu_group	*iommu_group;
> @@ -422,6 +423,7 @@ struct dev_iommu {
>  	void				*priv;
>  	u32				max_pasids;
>  	u32				attach_deferred:1;
> +	u32				shadow_on_flush:1;

This causes a merge conflict with a48ce36e2786f ("iommu: Prevent
RESV_DIRECT devices from blocking domains"), The resolution is trivial
though in that shadow_on_flush:1 can just be added after (or before)
require_direct:1. @Joro do you want me to sent a version with this
resolution regardless or will you resolve this when applying?

>  };
>  
>  int iommu_device_register(struct iommu_device *iommu,
>