[..] > Make a final call to direct_pte_prefetch_many() if there are "trailing" > SPTEs to prefetch, i.e. SPTEs for GFNs following the faulting GFN. The > call to direct_pte_prefetch_many() in the loop only handles the case > where there are !PRESENT SPTEs preceding a PRESENT SPTE. > > E.g. if the faulting GFN is a multiple of 8 (the prefetch size) and all > SPTEs for the following GFNs are !PRESENT, the loop will terminate with > "start = sptep+1" and not prefetch any SPTEs. > > Prefetching trailing SPTEs as intended can drastically reduce the number > of guest page faults, e.g. accessing the first byte of every 4kb page in > a 6gb chunk of virtual memory, in a VM with 8gb of preallocated memory, > the number of pf_fixed events observed in L0 drops from ~1.75M to <0.27M. > > Note, this only affects memory that is backed by 4kb pages as KVM doesn't > prefetch when installing hugepages. Shadow paging prefetching is not > affected as it does not batch the prefetches due to the need to process > the corresponding guest PTE. The TDP MMU is not affected because it > doesn't have prefetching, yet... Tested-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx> I ran some tests. - VM Boot up From EPT_VIOLATION 1192184 75.18% 4.40% 0.77us 18020.01us 4.32us ( +- 1.71% ) to EPT_VIOLATION 947460 69.92% 4.64% 0.69us 34902.15us 5.06us ( +- 1.64% ) - Running test app (in VM) From EPT_VIOLATION 6550167 71.05% 11.76% 0.77us 32562.18us 3.51us ( +- 0.36% ) to EPT_VIOLATION 5489904 68.32% 11.29% 0.71us 16564.19us 3.92us ( +- 0.29% )
