On Wed, Aug 14, 2024 at 07:10:31PM -0300, Jason Gunthorpe wrote: [...] > > Nope. KVM ARM does (see get_vma_page_shift()) but I strongly suspect that's only > > a win in very select use cases, and is overall a non-trivial loss. > > Ah that ARM behavior was probably what was being mentioned then! So > take my original remark as applying to this :) > > > > I don't quite understand your safety argument, if the VMA has 1G of > > > contiguous physical memory described with 4K it is definitely safe for > > > KVM to reassemble that same memory and represent it as 1G. > > > > That would require taking mmap_lock to get the VMA, which would be a net negative, > > especially for workloads that are latency sensitive. > > You can aggregate if the read and aggregating logic are protected by > mmu notifiers, I think. A invalidation would still have enough > information to clear the aggregate shadow entry. If you get a sequence > number collision then you'd throw away the aggregation. > > But yes, I also think it would be slow to have aggregation logic in > KVM. Doing in the main mmu is much better. +1. For KVM/arm64 I'm quite hesitant to change the behavior to PTE mappings in this situation (i.e. dump get_vma_page_shift()), as I'm quite certain that'll have a performance regression on someone's workload. But once we can derive huge PFNMAP from the primary MMU then we should just normalize on that. -- Thanks, Oliver
