Hi Gavin, On Tue, Feb 14, 2023 at 04:57:59PM +1100, Gavin Shan wrote: > On 2/7/23 3:58 AM, Ricardo Koller wrote: <snip> > > Eager Page Splitting fixes the above two issues by eagerly splitting > > huge-pages when enabling dirty logging. The goal is to avoid doing it > > while faulting on write-protected pages. </snip> > I'm not sure why we can't eagerly split the PMD mapping into 512 PTE > mapping in the page fault handler? The entire goal of the series is to avoid page splitting at all on the stage-2 abort path. Ideally we want to minimize the time taken to handle a fault so we can get back to running the guest. The requirement to perform a break-before-make operation to change the mapping granularity can, as Ricardo points out, be a bottleneck on contemporary implementations. There is a clear uplift with the proposed implementation already, and I would expect that margin to widen if/when we add support for lockless (i.e. RCU-protected) permission relaxation. > In the implementation, the newly introduced API > kvm_pgtable_stage2_split() calls to kvm_pgtable_stage2_create_unlinked() > and then stage2_map_walker(), which is part of kvm_pgtable_stage2_map(), > to create the unlinked page tables. This is deliberate code reuse. Page table construction in the fault path is largely similar to that of eager split besides the fact that one is working on 'live' page tables whereas the other is not. As such I gave the suggestion to Ricardo to reuse what we have today for the sake of eager splitting. -- Thanks, Oliver
