Re: [PATCH v6 5/6] iommu/dma: Allow a single FQ in addition to per-CPU FQs

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 2/15/23 7:03 AM, 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 more 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 can be
> selected as a flag bit in a new dma_iommu_options struct which can be
> modified from its defaults by IOMMU drivers implementing a new
> ops.tune_dma_iommu() callback. As a first user the s390 IOMMU driver
> selects the single queue mode if IOTLB flushes are needed on map which
> indicates shadow table use. 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/linux-iommu/3e402947-61f9-b7e8-1414-fde006257b6f@arm.com/
> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com> #s390