On Thu, 2023-01-19 at 16:55 +0100, Niklas Schnelle wrote: > On Wed, 2023-01-04 at 13:05 +0100, 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. > > > > Thus let's introduce a single flush queue mode IOMMU_DOMAIN_DMA_SQ that > > reuses the same queue logic but only allocates a single global queue > > allowing larger batches of IOVAs to be freed at once and with larger > > timeouts. This is to allow the common IOVA flushing code to more closely > > resemble the global flush behavior used on s390's previous internal DMA > > API implementation. > > > > As we now support two different variants of flush queues rename the > > existing __IOMMU_DOMAIN_DMA_FQ to __IOMMU_DOMAIN_DMA_LAZY to indicate > > the general case of having a flush queue and introduce separate > > __IOMMU_DOMAIN_DMA_PERCPU_Q and __IOMMU_DOMAIN_DMA_SINGLE_Q bits to > > indicate the two queue variants. > > > > Link: https://lore.kernel.org/linux-iommu/3e402947-61f9-b7e8-1414-fde006257b6f@xxxxxxx/ > > Signed-off-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx> > > --- > > v2 -> v3: > > - Rename __IOMMU_DOMAIN_DMA_FQ to __IOMMU_DOMAIN_DMA_LAZY to make it more clear > > that this bit indicates flush queue use independent of the exact queuing > > strategy > ---8<--- > > > > > - for (i = 0; i < IOVA_FQ_SIZE; i++) > > - INIT_LIST_HEAD(&fq->entries[i].freelist); > > + if (rc) { > > + pr_warn("iova flush queue initialization failed\n"); > > + return rc; > > } > > > ---8<--- > > > > mutex_unlock(&group->mutex); > > @@ -2896,10 +2900,10 @@ static int iommu_change_dev_def_domain(struct iommu_group *group, > > } > > > > /* We can bring up a flush queue without tearing down the domain */ > > - if (type == IOMMU_DOMAIN_DMA_FQ && prev_dom->type == IOMMU_DOMAIN_DMA) { > > + if (!!(type & __IOMMU_DOMAIN_DMA_LAZY) && prev_dom->type == IOMMU_DOMAIN_DMA) { > > ret = iommu_dma_init_fq(prev_dom); > > if (!ret) > > - prev_dom->type = IOMMU_DOMAIN_DMA_FQ; > > + prev_dom->type = type; > > Here domain->type is set only after calling iommu_dma_init_fq(). Actually I think even in the current code the above and the similar code in iommu.c isn't ideal. When going from DMA to DMA-FQ with a bound driver the flush queue is used from the moment that WRITE_ONCE(cookie- >fq_domain, domain) executes in iommu_dma_init_fq() so there is a window where the flush queue is already used but domain->type is still DMA. By adding a type parameter to iommu_dma_init_fq() we can set domain->type before the WRITE_ONCE() and thus close this window and it even makes the callsites of iommu_dma_init_fq() simpler. > > > goto out; > > } > > >
