> Am 17.12.2020 um 20:04 schrieb David Hildenbrand <da...@redhat.com>: > > On 17.12.20 18:55, Alex Williamson wrote: >>> On Wed, 16 Dec 2020 15:11:54 +0100 >>> David Hildenbrand <da...@redhat.com> wrote: >>> >>> Let's query the maximum number of DMA mappings by querying the available >>> mappings when creating the container. >>> >>> In addition, count the number of DMA mappings and warn when we would >>> exceed it. This is a preparation for RamDiscardMgr which might >>> create quite some DMA mappings over time, and we at least want to warn >>> early that the QEMU setup might be problematic. Use "reserved" >>> terminology, so we can use this to reserve mappings before they are >>> actually created. >> >> This terminology doesn't make much sense to me, we're not actually >> performing any kind of reservation. > > I see you spotted the second user which actually performs reservations. > >> >>> Note: don't reserve vIOMMU DMA mappings - using the vIOMMU region size >>> divided by the mapping page size might be a bad indication of what will >>> happen in practice - we might end up warning all the time. >> >> This suggests we're not really tracking DMA "reservations" at all. >> Would something like dma_regions_mappings be a more appropriate >> identifier for the thing you're trying to count? We might as well also > > Right now I want to count > - Mappings we know we will definitely have (counted in this patch) > - Mappings we know we could eventually have later (RamDiscardMgr) > >> keep a counter for dma_iommu_mappings where the sum of those two should >> stay below dma_max_mappings. > > We could, however, tracking active IOMMU mappings when removing a memory > region from the address space isn't easily possible - we do a single > vfio_dma_unmap() which might span multiple mappings. Same applies to > RamDiscardMgr. Hard to count how many mappings we actually *currently* > have using that approach. >
Thinking about it, might actually be possible when tracking active mappings per iommu / ram discard mgr as well. Will have a look in the new year - thanks.
