On Fri, Aug 01, 2025, James Houghton wrote: > On Fri, Aug 1, 2025 at 7:11 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > On Mon, Jul 28, 2025, James Houghton wrote: > > > On Wed, Jul 23, 2025 at 2:04 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > On Mon, Jul 07, 2025, James Houghton wrote: > > > > Right, but we also don't want to wait for the initial fault-in either, no? I.e. > > > > plumbing in MAP_POPULATE only fixes the worst of the delay, and maybe only with > > > > the TDP MMU enabled. > > > > > > > > In other words, it seems like we need a helper (option?) to excplitly "prefault", > > > > all memory from within the guest, not the ability to specify MAP_POPULATE. > > > > > > I don't want the EPT to be populated. > > > > > > In the event of a hugepage being executed, consider another memory > > > access. The access can either (1) be in the executed-from hugepage or > > > (2) be somewhere else. > > > > > > For (1), the optimization in this series doesn't help; we will often > > > be stuck behind the hugepage either being destroyed or reconstructed. > > > > > > For (2), the optimization in this series is an improvement, and that's > > > what this test is trying to demonstrate. But this is only true if the > > > EPT does not have a valid mapping for the GPA we tried to use. If it > > > does, the access will just proceed like normal. > > > > > > This test only times these "case 2" accesses. Now if we didn't have > > > MAP_POPULATE, then (non-fast) GUP time appears in these results, which > > > (IIRC) adds so much noise that the improvement is difficult to > > > ascertain. But with MAP_POPULATE, the difference is very clear. > > > > Oh, right, the whole point is to measure fault-in performance. > > > > In that case, rather than MAP_POPULATE, can we do the slightly more standard (for > > VMMs) thing of writing (or reading) memory from host userspace? I don't think it's > > worth plumbing in extra_mmap_flags just for MAP_POPULATE, in no small part because > > MAP_POPULATE is effectively best effort, and doesn't work for VM_PFNMAP (or VM_IO). > > > > Those quirks shouldn't matter for this case, and _probably_ won't ever matter for > > any KVM selftest, but it's enough to make me think MAP_POPULATE is a pattern we > > don't want to encourage. > > What if vm_mem_add() just returned the VA of the added region, and > then the test could call mlock() on it? I like it!
