On 5/11/2018 3:06 PM, Jean-Philippe Brucker wrote: > PCIe devices can implement their own TLB, named Address Translation Cache > (ATC). Enable Address Translation Service (ATS) for devices that support > it and send them invalidation requests whenever we invalidate the IOTLBs. > > Range calculation > ----------------- > > The invalidation packet itself is a bit awkward: range must be naturally > aligned, which means that the start address is a multiple of the range > size. In addition, the size must be a power of two number of 4k pages. We > have a few options to enforce this constraint: > > (1) Find the smallest naturally aligned region that covers the requested > range. This is simple to compute and only takes one ATC_INV, but it > will spill on lots of neighbouring ATC entries. > > (2) Align the start address to the region size (rounded up to a power of > two), and send a second invalidation for the next range of the same > size. Still not great, but reduces spilling. > > (3) Cover the range exactly with the smallest number of naturally aligned > regions. This would be interesting to implement but as for (2), > requires multiple ATC_INV. > > As I suspect ATC invalidation packets will be a very scarce resource, I'll > go with option (1) for now, and only send one big invalidation. We can > move to (2), which is both easier to read and more gentle with the ATC, > once we've observed on real systems that we can send multiple smaller > Invalidation Requests for roughly the same price as a single big one. > > Note that with io-pgtable, the unmap function is called for each page, so > this doesn't matter. The problem shows up when sharing page tables with > the MMU. > > Timeout > ------- > > ATC invalidation is allowed to take up to 90 seconds, according to the > PCIe spec, so it is possible to hit the SMMU command queue timeout during > normal operations. > > Some SMMU implementations will raise a CERROR_ATC_INV_SYNC when a CMD_SYNC > fails because of an ATC invalidation. Some will just abort the CMD_SYNC. > Others might let CMD_SYNC complete and have an asynchronous IMPDEF > mechanism to record the error. When we receive a CERROR_ATC_INV_SYNC, we > could retry sending all ATC_INV since last successful CMD_SYNC. When a > CMD_SYNC fails without CERROR_ATC_INV_SYNC, we could retry sending *all* > commands since last successful CMD_SYNC. > > We cannot afford to wait 90 seconds in iommu_unmap, let alone MMU > notifiers. So we'd have to introduce a more clever system if this timeout > becomes a problem, like keeping hold of mappings and invalidating in the > background. Implementing safe delayed invalidations is a very complex > problem and deserves a series of its own. We'll assess whether more work > is needed to properly handle ATC invalidation timeouts once this code runs > on real hardware. > > Misc > ---- > > I didn't put ATC and TLB invalidations in the same functions for three > reasons: > > * TLB invalidation by range is batched and committed with a single sync. > Batching ATC invalidation is inconvenient, endpoints limit the number of > inflight invalidations. We'd have to count the number of invalidations > queued and send a sync periodically. In addition, I suspect we always > need a sync between TLB and ATC invalidation for the same page. > > * Doing ATC invalidation outside tlb_inv_range also allows to send less > requests, since TLB invalidations are done per page or block, while ATC > invalidations target IOVA ranges. > > * TLB invalidation by context is performed when freeing the domain, at > which point there isn't any device attached anymore. > > Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@xxxxxxx> Nothing specific about this patch but just a general observation. Last time I looked at the code, it seemed to require both ATS and PRI support from a given hardware. I think you can assume that for ATS 1.1 specification but ATS 1.0 specification allows a system to have ATS+PASID without PRI. QDF2400 is ATS 1.0 compatible as an example. Is this an assumption / my misinterpretation? -- Sinan Kaya Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
